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A  PRACTICAL  TREATISE 
ON  FOUNDATIONS, 


EXPLAINING  FULLY  THE  PRINCIPLES 
INVOLVED. 


WITH 

DESCRIPTIONS  OF  ALL  OF  THE  MOST  RECENT  STRUCTURES ,  AC¬ 
COMPANIED  BY  NUMEROUS  DRAWINGS ;  ALSO  AN  ACCU¬ 
RATE  RECORD  OF  THE  BEARING  RESISTANCES 
OF  MATERIALS  ASDETERMINED  FROM  THE 
LOADS  OF  ACTUAL  STRUCTURES. 


BY 


W.  M.  PATTON,  C.E., 

Formerly  Professor  of  Engineering  at  the  Virginia  Military  Institute  ; 

Engineer  in  charge  of  the  Mobile  River ,  Ohio  River ,  Szisquehantia  River ,  and  Schuyl¬ 
kill  River  Bridges  ;  late  Chief-Engineer  of  the  Mobile  and  Birmingham 
Railway  and  of  the  Louisville ,  St.  Louis,  and  Texas  Railway. 


FIRST  EDITION. 

FIRST  THOUSAND. 


NEW  YORK : 

JOHN  WILEY  &  SONS, 

53  East  Tenth  Street 
1893. 


Copyright,  1893, 

BY 

W.  M.  PATTON. 


Robert  Drummond, 

Electrotyper , 
tM  and  44R  Pearl  St,f 
New  York. 


PREFACE. 


In  a  work  on  Foundations,  theories  and  formulae  are  of 
little  value  ;  therefore  but  little  space  is  given  to  the  discussion 
or  criticism  of  either.  The  more  common  formulae  are  given 
without  any  attempt  to  explain  the  laws  or  premises  upon 
which  they  are  based  ;  a  few  examples  are  worked  out  in  order 
to  show  the  actual  or  relative  values  of  the  terms  entering  into 
them,  and  to  compare  the  results  with  those  used  in  practice. 
I  do  not  do  this  either  to  ignore  or  underrate  the  value  or  im¬ 
portance  of  theoretical  investigations;  if  the  formulae  deduced 
in  themselves  do  not  have  practical  value,  they  incidentally 
lead  to  comparisons  with  actual  results,  and  induce  the  publica¬ 
tion  of  a  large  mass  of  more  or  less  accurate  records  of  observed 
facts.  Theory  and  practice  should  go  hand  in  hand  ;  but  it 
is  to  be  regretted  that  in  many  institutions  claiming  to  be 
schools  of  engineering  so  great  a  preponderance  in  time  and 
energy  is  given  to  the  theoretical  side  of  the  question,  even 
almost  to  the  exclusion  of  practical  instruction,  whereby  many 
erroneous  ideas  and  principles  are  instilled  into  the  minds 
of  young  engineers,  to  eradicate  which  years  of  labor,  blun¬ 
dering,  and  mortification  are  required,  causing  loss  and  delays 
to  their  employers,  loss  and  injustice  to  contractors  by  onerous 
and  useless  requirements  and  exactions,  which  could  have  been 
saved  by  a  knowledge  of  a  few  facts  and  methods  found  in 


IV 


PREFACE. 


common  and  every-day  practice;  theorists  claiming  that  the 
“costs  of  labor,  materials  and  construction,  and  also  rules  of 
practice  ”  are  of  no  value  to  the  student  of  engineering,  as 
these  will  be  acquired  after  leaving  college,  and  that  “  prin¬ 
ciples  alone  are  necessary  to  be  taught.” 

Having  been  a  professor  for  over  six  years,  I  have  fully 
realized  the  need  of  suitable  books,  by  which  I  could  temper 
the  almost  painfully  scientific,  abstruse,  and  purely  theoretical 
books  that  I  was  compelled  to  put  into  the  hands  of  the 
student  in  engineering,  which  could  only  be  partially  supple¬ 
mented  from  a  few  years’  prior  experience  in  active  practice, 
during  which  the  full  force  of  what  I  have  stated  above  was 
full)7  realized. 

With  the  above  experience  and  the  experience  derived  from 
eighteen  years  of  active  practice,  a  very  large  portion  of  which 
was  devoted  to  bridge  construction  in  many  parts  of  the 
United  States,  building  on  a  great  variety  of  soils,  necessarily 
requiring  a  great  variety  in  the  designs  and  methods  of  con¬ 
struction,  I  have  undertaken  to  write  the  following  pages. 
The  descriptive  portions  of  this  volume  have  been  to  a  large 
extent  based  upon  my  own  experience,  the  facts  of  which  are 
taken  from  records  made  at  the  time  and  still  in  my  possession  ; 
they  can  therefore  be  relied  upon  as  accurate.  The  drawings, 
with  few  exceptions,  are  taken  from  my  own  designs,  and  are 
accurate  representations  of  the  actual  structures  used ;  in  these 
my  only  aim  was  simplicity  in  design,  convenience  in  construc¬ 
tion,  combined  with  cheapness,  strength,  and  suitableness  for 
the  purpose  in  view.  Unusual  sizes  and  shapes  of  the  parts  were 
studiously  avoided,  as  only  adding  to  the  cost  of  material  and 
construction  without  any  compensating  practical  advantages. 

I  have  only  given  prominence  to  these,  as  I  believed  they  can 
be  fairly  well  taken  as  typical  designs,  and  with  a  few  modifica¬ 
tions  in  the  details  can  be  readily  converted  into  the  designs  of 
other  engineers  for  the  same  purposes.  Full  descriptions,  how- 


PREFA  CE. 


V 


•ever,  have  been  given  of  all  of  the  latest  and  largest  structures, 
which  can  be  readily  understood  when  taken  in  connection 
with  the  drawings  given.  I  have  collected  from  all  available 
sources  facts  in  connection  with  this  all-important  subject 
that  have  been  published  up  to  the  present  date,  such  as  the 
actual  loads  and  pressures  on  every  variety  of  material,  accu¬ 
rate  descriptions  of  all  designs  and  methods  of  construction,  all 
useful  knowledge  of  the  qualities,  properties,  and  strength  of 
the  materials  used.  Believing  that  the  want  of  familiarity  with 
the  costs  of  materials  and  construction,  the  usual  dimensions 
and  forms  of  parts,  and  the  quantities  of  materials  required  in 
the  more  common  structures,  as  expressed  in  bills  of  material 
and  records  of  actual  and  comparative  costs  of  structures,  is  a 
most  fruitful  soures  of  waste  of  money  in  making  contracts,  as 
designing  contractors,  by  magnifying  the  costs  of  materials  and 
•construction,  and  the  difficulties  and  risks  to  be  incurred,  im¬ 
pose  upon  the  credulity,  ignorance  and  fears  of  engineers,  there¬ 
by  securing  enormous  profits  on  their  works,  for  these  reasons 
I  have  devoted  more  than  the  usual  space  to  these  matters. 
I  have  expressed  opinions,  made  suggestions  and  (I  hope) 
kindly  criticisms,  knowing  full  well  that  if  they  are  erroneous 
or  not  justified  by  the  facts  presented  they  will  be  corrected, 
for  which  kindness  I  desire  to  express  my  thanks  in  ad¬ 
vance.  No  one  need  be  misled  by  opinions,  as  the  facts 
are  present  in  full.  I  have  endeavored  in  writing  this  volume 
to  confine  myself  as  closely  as  possible  to  matters  pertain¬ 
ing  to  the  subject  of  foundations,  by  which  I  mean  those 
parts  of  structures  resting  on  and  directly  supported  by  the 
materials  of  the  earth,  and  these  materials  themselves  in  regard 
to  their  capacity  to  support  the  loads  or  pressures  resting  upon 
them.  There  has  always  been  some  confusion  as  to  the  mean¬ 
ing  of  the  term  Foundation:  it  is  difficult,  if  not  impossible, 
to  separate  that  which  supports  a  pressure  from  that  which 
produces  it.  We  must  know  the  magnitude,  the  direction,  and 


VI 


PREFACE. 


\ 


the  point  of  application  of  a  force;  and  all  three  must  be 
known.  If  the  force  is  distributed,  we  must  know  the  nature 
of  the  distribution,  whether  uniform,  uniformly  varying,  or  ir¬ 
regularly  varying,  so  as  to  provide  proper  supports  and  resist¬ 
ances,  with  the  requisite  strength  and  at  the  required  points. 
Except  in  so  far  as  these  considerations  enter,  I  think  that  I 
have  confined  myself  within  the  limits  of  the  subject.  To 
avoid  confusion  or  too  much  repetition,  I  will  always  call  the 
natural  materials,  of  whatever  nature,  upon  which  the  structure 
is  founded  or  built,  the  Foundation-beds  ;  all  else  will  be  called 
Foundations  or  Substructures,  these  being  the  parts  of  the 
structure  under  the  surface  of  the  ground  or  water.  Those 
portions  above  are  only  described  or  illustrated  where  it  could 
not  be  avoided  either  for  a  clearer  understanding  or  for  sake  of 
valuable  comparisons.  Where  tables  and  other  data  have  been 
taken  from  books,  I  have  endeavored  to  give  the  authors  the 
credit  in  the  description.  I  am,  however,  largely  indebted  to 
the  editors  of  the  Engineering  News,  who  kindly  granted  the 
free  use  of  the  columns  of  their  valuable  and  wide-awake 
magazine.  I  am  also  under  obligations  to  Mr.  C.  A.  Brady. 
C.E. ;  Mr.  I.  E.  A.  Rose,  architect ;  and  Professor  R.  A.  Marr, 
of  the  Virginia  Military  Institute,  for  valuable  aid  in  prepar¬ 
ing  the  drawings.  I  have  been  greatly  assisted  in  other  ways 
by  Col.  E.  W.  Nichols,  Prof,  of  Mathematics,  Virginia  Military 
Institute. 

The  volume  is  divided  into  three  parts;  it  is  further  sub¬ 
divided  into  articles  and  paragraphs.  The  articles  are  numbered 
continuously  throughout  the  volume,  the  paragraphs  are  only 
numbered  continuously  through  each  part.  Par.  I  is  at  the 
beginning  of  each  part. 

W.  M.  Patton,  C.E. 

Lexington,  Va.,  May,  1893. 


TABLE  OF  CONTENTS. 


PART  FIRST. 

ARTICLE  I. 

PAGE 

Foundation-beds: — Of  rock,  clay,  sand,  gravel,  and  silt — Rules  for  prepa¬ 
ration  of — Bearing  resistance  of — Practical  deductions — Tables  of 
resistance  to  crushing  of  stone— Practical  determination  of  bearing 
resistance  of — Failure  of  structure  mainly  due  to  defective  .  .  1 

ARTICLE  II. 

Foundations: — Means  adopted  in  constructing — Of  concrete— Composi¬ 
tion  of  concrete — Methods  of  mixing — Consistency  of  mortar  in — 
Proportions  in,  of  stone  and  mortar— Methods  of  mixing  for  the  Ohio, 
Susquehanna,  Schuylkill,  and  Tombigbee  river  bridges — Kinds  of 
stone  suitable — Broken  bricks  and  shells  in — Rules  and  principles  in 
making — Proportions,  mixing,  etc.,  under  Washington  Monument — 
Absolute  rules  for  proportion  of  quantities  useful  and  practicable 
in  certain  cases — Impracticable  when  handling  large  quantities  with 
limited  time  and  money  available . 9 

ARTICLE  III. 

Concrete: — Uses  and  advantages  of — Under  walls  of  houses,  bridge  piers 
and  abutments,  and  retaining- walls — Crushing  strength  of  .  .  80 

ARTICLE  IV. 

Building  Stones: — Granite,  marble, limestone,  slate, and  sandstone — Prop¬ 
erties  of,  structural  and  chemical — Stratified  and  unstratified — Quarry 
indications  as  to  quality — Siliceous,  calcareous,  argillaceous — Stones 
that  harden  by  exposure,  stones  that  disintegrate  or  deteriorate  on 
exposure — Resistance  to  acid  and  atmospheric  influences — Resistance 

vii 


TABLE  OF  CONTENTS. 


viii 


PAGE 

of,  to  heat — Capacity  of  absorbing  water — Durability  of,  and  suitable¬ 
ness  for  building  purposes . .  .  23 

ARTICLE  V. 

Quarrying: — Rules  and  principles — Drilling  by  hand  and  by  machinery— 
Economical  conditions  of— Blasting  with  powder  and  dynamite,  pre¬ 
cautions  necessary  to  avoid  injury  to  stones  in — For  face  stones,  for 
backing,  rubble  and  concrete . 28 

ARTICLE  VI. 

Stereotomy: — Only  simple  forms  required  for  ordinary  work— Useful  for 
architectural  and  ornamental  purposes,  requiring  complicated  forme 
and  shapes — Tools  used — Methods  used  in  cutting  and  dressing  ordi¬ 
nary  stones — Requirements  as  to  beds  and  joints— Examination  and 
inspection  of  stone — Chisel-drafts,  pitch-lines — Models  and  templets — 
Necessary  and  unnecessary  requirements  .  ...  34 

ARTICLE  VII. 

Masonry: — Stones  used  in — Granite,  marble,  limestone,  and  sandstone — 
Classified — Dry  stones,  rough  rubble,  rubble  in  courses, block-in-course, 
ashlar — Stone  suitable  for — Relative  dimensions  of  stone  in — Facing 
and  backing — Headers  and  stretchers — Dimensions  and  proportions — 
Bond  in — Bed  and  side  joints  in — Ashlar,  rubble,  and  concrete  back¬ 
ing  compared — Grouting  walls  of — Footing-courses,  neat  work,  string¬ 
courses  and  coping,  raising  stones— Appearance  of  masonry  on  face 
no  indication  as  to  kind  or  quality  of — Uses  and  advantages  of  chisel- 
drafts  and-pitch-lines — Proper  position,  length,  and  other  dimensions 
of  headers . .  .  97 

ARTICLE  yin. 

String-courses  and  Coping: — Uses  of — Projection  of — Kind  of  masonry 
in — Position  of — Shape  of  piers  in  plan— Square,  circular,  elliptic,  and 
triangular  ends— Templets  for — How  laid  and  constructed — For  what 
purpose — Proportions  of  length  and  breadth  of  ends — Cutwater  or 
starling  proper . 46 


ARTICLE  IX. 

Ice  and  Wind  Pressures: — Velocity  and  force  of  wind — Formula  for — 
How  estimated  on  piers  and  trusses — Pressure  of  ice — Force  of  wind — 
How  estimated— Various  theories  and  assumptions — Moment  of  over¬ 
turning  forces  on  trusses  and  piers — Moment  of  resistance  to  over- 


TABLE  OF  CONTENTS. 


IX 


turning— Ice  and  drift  gorges — Effect  of,  on  structures — Dimensions 
of  piers  required  determined  by  other  conditions,  and  always  suffi¬ 
cient  to  resist  these  external  and  unusual  pressures — Examples  given,  49 

ARTICLE  X. 

Retaining- Walls: — Stability  of  masses  of  earth,  frictional — Cohesion 
and  adhesion  destroyed  by  exposure— Angle  of  repose — Natural  slope 
— Uses  of — Resultant  pressure  on — Magnitude,  direction,  and  point  of 
application  of  resultant — Moment  of  overturning  force — Moment  of 
stability  or  resistance  to  overturning — Resistance  to  sliding  or  fric¬ 
tional  stability — Formulae  and  practical  rules  for  thickness  of  walls — 
Pressure  of  water  or  quicksand  on,  and  thickness  required — Con¬ 
struction  of — Kind  of  masonry  in — Face,  backing,  and  footing  courses 
— Position  of  centre  of  pressure  or  resistance  with  respect  to  centre  of 
figure  of  base — Plan  and  Section — U,  T,  and  wing  abutments  and 
walls . 52 


ARTICLE  XI. 

Retaining- Walls  Formulae  for  stability  —  Rankine’s,  Trautwine’s, 

Moseley’s— Practical  examples  and  rules . 57 

ARTICLE  XII. 

Arches: — Theory  of — Mathematical  and  graphical  methods — Depths  of 
arch-ring  at  keystone  and  springing— Centres  of  pressure — Lines  of 
pressure — Masonry  in  arch-ring — Abutments— Spandrel  walls — Back¬ 
ing — Flat  and  pointed  arches,  manner  of  giving  way — Backing  to 
prevent  failure — Stability  of — Resistance  to  crushing,  overturning, 
sliding— Definition  of  terms  used — Full  centre — Elliptical  and  seg¬ 
mental  . 64 


ARTICLE  XIII. 

Skew  Arches: — Definition  of — String-course  and  ring-course  joints — De¬ 
velopment  of  soffit — Usual  construction  of . 72 

ARTICLE  XIV. 

Arches: — Formulae  for  depth  of  keystone— Examples  under — Lines  of 
pressure . 73 


ARTICLE  XV. 

Brick:— Brick  walls  and  piers— Brick-making— Uses  and  advantages  of— 
English  and  Flemish  bond— Construction  and  strength  of— Durabil- 


X 


TABLE  OF  CONTENTS. 


ity  of — Stability  of — Failure  of — Mortar,  adhesion  to — Use  below 
ground  or  water  not  recommended  unless  cement  mortar  is  used — 
Slate  between  courses — Importance  of  being  kept  wet — Thickness  of 
walls — Compressed  brick — Dimensions — Measurement  of — Sewers — 
Pavements  .  . 75 


ARTICLE  XVI. 

Brick  Arches: — Usually  built  in  rings — Headers  should  be  used— Two 
methods  of  building— Thickness  of  joints  at  intrados  and  extrados — 
Slate  in  joints — Use  and  advantage  of  hoop-iron — Used  in  lining  tun¬ 
nels — Thickness  of  lining— Uses  of  —Stability  of — How  estimated  and 
paid  for  . . 8£ 


ARTICLE  XVII. 

Arches: — Summary  of  theories  and  their  practical  applications— External 
forces — Assumptions  made — Lines  of  pressure — Precautions  necessary 
in  constructing — Centres  for  arches  .  . . 85 

ARTICLE  XVIII. 

Box  Culverts:— Uses,  dimensions,  and  construction  of — Kind  of  masonry 
in— Thickness  of  walls — Height — Covering  stones — Precautions  in 
tilling  over  aud  around  culverts  and  arches — Rules  and  principles  of 
masonry  construction  ..........  8® 


ARTICLE  XIX. 

Cements  and  Hydraulic  Limes: — How  and  where  obtained — Kind  of 
stones— Percentage  of  lime,  clay,  or  silica, etc. — Temperature  required 
in  burning — Mixture  of  different  grades  of  stone — General  properties 
and  qualities — Portland,  heavy  slow-setting — Rosendales,  light  quick¬ 
setting — Hydraulic  activity  and  energy — Set  not  well  defined — How 
determined — Proportions  sand,  cement,  and  water — Tensile  strength 
of — Requirements  of — Simple  tests  as  to  fineness,  set,  etc. — Slake 
slowly — Quick  lime  obtained  from  pure  carbonates,  or  those  contain¬ 
ing  small  per  cents  of  clay,  silica,  etc.,  and  will  not  harden  under 
water — Process  of  slaking— Mixing  with  water — Quantities  of  mortar 
obtained  per  barrel  of  cement  aud  lime — Quantity  required  in  mason¬ 
ry  and  concrete — Proportions  of  sand  and  water  in  mortars.  .  .  91 

ARTICLE  XX. 

Mortar: — Definition  of — Proportions  of  cement,  or  lime,  sand,  and  water — 
Proportions  of,  in  masonry — Cement  and  lime  mixed  economical — 

Test  of  quick-lime  and  slaking  of— Cement  and  quick-lime  stones — 


'TABLE  OF  CONTENTS. 


Chemical  and  mechanical  compositionof — Percentage  of  beneficial  and 
injurious  ingredients — Proper  cements  to  be  used — Tests  of — Brands 
of — Tensile  tests  of  briquettes— Hardening  of  lime  and  cement  mor¬ 
tars— Deposited  under  water  should  have  some  set  first,  unless 
deposited  in  bags — Lime  mortars  not  used  under  water — Pozzuolaua — 
Definition  and  uses  of — Freezing  of  mortars  not  considered  injurious 
Experiments  on — Salt  in  mortars — Pointing  mortars.  Also  see  Sup¬ 
plement  . 96 


ARTICLE  XXI. 

Sand: — Uses  of,  in  mortar — Proportions — Qualities  necessary — Sizes  of 
grains — Tests  for  cleanness  and  sharpness — Salt-water  sand — River 
and  pit  sand — Cleanness  and  sharpness  of  grain  most  important 
requirements . 106 


ARTICLE  XXII. 

Stability  of  Piers: — External  pressures — Current,  ice,  drift,  and  wind — 
Expansion  and  contraction  of  ice — Effects  on  piers — Ice  and  drift 
gorges,  and  flow  of— Destructive  effects  of — Tearing  and  crushing 
resistance  of  ice — Protection  of  piers,  cutwaters,  etc . 107 

ARTICLE  XXIII. 

Water-way  in  Culverts: — Formulae  for— Practical  rules — Dimensions 
of,  and  how  determined  practically — Terra-cotta  pipes— Iron  pipes — 
Uses  of  culverts . 113 

ARTICLE  XXIV. 

Arch  Culverts: — Dimensions  and  construction  of — Thickness  of  abut¬ 
ments — Formulae  for — Practical  examples — Lengths  of  span — Plans 
and  sections — Surcharged  walls — Formula  for — General  remarks  .  115 

ARTICLE  XXV. 

Cost  of  Work: — Remarks  on — Brick-walls  and  piers — Trestle  work, 
framed  and  pile — Timber,  masonry,  caissons,  cribs  and  coffer-dams — 
Conditions  in  contracts . .  120 


ARTICLE  XXVI. 

Cost  of  Work: — Tables  of — Cost  of  quarrying,  cutting,  laying,  sand, 
masonry,  brick-work,  rubble,  concrete,  paving,  brick,  arch  stones, 
cement,  lime,  etc . .  123 


TABLE  OF  CONTENTS. 


xii 


ARTICLE  XXVII. 

IAGB 

Dimensions,  Quantities  and  Cost: — Examples — Ohio,  Susquehanna, 
Schuylkill,  Tombigbee  river  bridges— Tables  of  quantities  and  costs 
— Cost  of  sinking  caissons  as  usually  estimated,  also  by  cubic  yard 
of  displacement . 126 

ARTICLE  XXVIII. 

Definitions  and  Tables: — Of  parts  of  arches,  piers,  and  retaining-walls, 
etc. — Tables — Of  resistance  to  crushing,  tearing,  cross-breaking — Of 
-weights  per  cubic  foot  of  materials — Of  angles  of  repose — Of  various 
materials — Of  bearing  resistance  of  soils— Uses  of,  and  practical  ex¬ 
amples  . .  136 


PART  SECOND. 

ARTICLE  XXIX. 

Timber  Foundations: — Why  used— Under  walls  of  houses — How  con¬ 
structed— Under  New  Orleans  custom-house — Under  towers — Unit 
pressures  should  be  same  under  all  parts— Piles  often  preferred  in  soft 
and  silty  soils— Cribs  and  grillages  under  piers — Construction  of — 
Sinking  of — Dangers  of  beds  of  sand  and  gravel — But  often  used — 
Example,  Parkersburg  bridge— Cribs  sunk  on  rock — Precautions  to  be 
taken — Cribs  sunk  on  rock— Coffer-dams  of  earth,  and  dimensions 
of — Remarks  on .  .  147 


ARTICLE  XXX. 

Coffer  Dams  of  Timber: — Double  walls  with  clay-puddle — How  con¬ 
structed —  Dimensions  —  Remarks  on — Single  wall  coffer-dams, 
tongued  and  grooved — Construction  of — With  vertical  timbers — 
With  timbers  and  plank  in  horizontal  layers— Crib  coffer  dams 
— Construction  uses  and  advantages— Puddle  for— Pumps — Exca¬ 
vation — Size  of  dam  important — Bracing— Precautions  for  safety — 
Coffer-dams  with  inner  cribs — Construction — Uses  and  advantages  of 
— Examples — Preparation  of  foundation-beds . 151 

ARTICLE  XXXI. 

Open  Caissons:— Construction  of— Preparation  of  bed  by  dredging— By 
piles_For  what  depths  useful  and  economical — Sides  of,  single  wall 
coffer-dams,  and  removable— Bottom  crib  or  grillage  forms  part  of 


TABLE  OF  CONTENTS. 


xin 


PAGE 

permanent  foundations — Generally  simply  resting  on  bed — Can  be 
secured  if  necessary . 162 


ARTICLE  XXXII. 

Cushing  Cylinder  Piers : — Construction  and  uses— Piles  actual  supports 
—Cylinder  casings  for  concrete — Use  of  concrete — Depths  sunk — 
Manner  of  sinking — Piers  wanting  in  stability — Are  economical,  and 
often  used — Cylinders  often  sunk  without  piles — Require  constant 
watching  and  large  quantities  of  riprap— Examples — Tensas  River 
bridge — Full  description  and  dimensions — Sinking— Shell  concrete 
used-— Contract  prices  for,  etc . 164 

ARTICLE  XXXIII. 

Sounding  and  Borings: — Importance  of— Common  neglect  of — Making 
—Errors  and  waste  resulting  from  neglect  of — First  method,  driving 
solid  rods,  uncertain,  unreliable,  and  unsatisfactory— Second  method, 
sinking  large  terra-cotta  or  iron  pipes  more  satisfactory,  but  more  or 
less  uncertain— Third  method,  sinking  small  iron  pipes  by  water-jet 
and  force-pump,  rapid,  economical,  reliable,  and  satisfactory— Descrip¬ 
tion  of  processes  in  sand  and  gravel  and  silt.  Also  see  Supplement  .  166 

ARTICLE  XXXIV. 

Timber  Pierr:— -Construction  and  uses— Ad  vantages  and  disadvantages  .  170 

ARTICLE  XXXV. 

Framed  Trestles:— Construction  and  uses — Designs  and  dimension  of 
parts . .  172 


ARTICLE  XXXVI. 

Properties  of  Timber: — Kinds  commonly  used — Pine,  oak,  cypress — 
Effects  of  bleeding  or  turpentining  pine-trees . 177 

ARTICLE  XXXVII. 

Durability  of  Timber: — Defects  of,  cracks,  shakes,  crippling,  dotimess, 
sponginess,  decay,  and  rot  where  developed  in  frame  structures 
—Examinations  for — Repairs  and  renewals— General  remarks  .  .  180 

.  ARTICLE  XXXVIII. 


Preservation  of  Timber:— Character  of  defects— Effect  of,  on  timber — 
Natural  seasoning — Protection  of  bridge  trusses — Artificialseasoning — 


XIV 


TABLE  OF  CONTENTS. 


Preservation  by  solutions  of  metallic  salts,  creosoting,  vulcanizing — 
Durability  as  affected  by  time  of  cutting  down  and  by  age  of  trees — 
Constantly  immersed  in  water,  favorable  for  durability — Asphalt  and 
other  paints — Discussions  and  remarks.  Also  see  Supplement  .  .  182 

ARTICLE  XXXIX. 

Framed  Trestles: — Two  types  not  often  used,  but  good  designs — 
Joints,  weak  points  in . 187 


ARTICLE  XL. 

Joints  and  Fastenings:— Mortise-aud-tenon— Disadvantage  of— Square 
abutting  joints  with  iron  straps — Advantages  of — Dovetail  joints 
— Strut  and  tie — Longitudinal  bracing— Fish  and  scarf  joints — 

Uses  and  designs — Actual  and  relative  strength  of  joints — Formulae 
and  examples  of  relative  resistance  to  crushing,  tearing,  and  shearing — 
Strength  of  connections  should  equal  strength  of  main  parts — Weak¬ 
est  part  determines  strength  of  entire  structure — Joints  in  king  and 
queen  trusses — Rules  and  principles  to  be  followed  in  all  joints  and 
fastenings— Joints  for  lengthening — Ties,  struts,  ties  and  struts,  and 
beams . 188 


ARTICLE  XLI. 

Trestle  Foundations: — Mud-sills— Masonry  pedestals — Piles — Advan¬ 
tages  and  disadvantages  of — Framed  trestles  divided  into  four  classes 
— Comparative  strength  and  economy  of  construction — Kind  of  stresses 
on  main  members— How  connected  to  resist — Formulas  for  and 
examples  of  relative  and  actual  strength  and  dimensions — Posts,  caps, 
sills,  stringers,  struts,  and  braces — Explanation  and  use  of  formulae — 
Tables  of  resistance  to  crushing,  tearing  and  cross-breaking — Ulti¬ 
mate  and  working  stresses,  factors-of -safety — Formula  for  long  columns 
— Bill  of  timber  and  iron  for  four  story  trestle  .  .  .  .  196 

ARTICLE  XLII. 

Timber  Piles: — Uses  of — Long  and  short — Kind  of  timber  used,  and  com¬ 
parative  value — Preparing  piles  for  driving — Squaring  butt — Pointing 
end — Square  ends  preferred  in  driving — Method  of  driving — Precau¬ 
tions  to  prevent  splitting — Excessive  brooming — Bands  and  shoes  of 
iron — Remarks  on  driving — Great  damage  to  piles  in  driving — Useless 
hammering  on  piles — Value  of  formulae  discussed — Reliance  mainly 
on  experiment  and  experience — Experiments  on  bearing  power  of 
piles  (also  see  Supplement) — Practical  conclusions — Peculiarities  in 


TABLE  OF  CONTLNTZ. 


XV 


driving  in  different  soils— Remarks — Usual  formulae  and  examples 
under  them — Rankine's,  Trautwine’s,  banders’ . 207 

ARTICLE  XLIII. 

Timber  Piles  -.—Engineering  News  formula — Latest  and  doubtless  the 
best — No  formula  considered  of  practical  value  depending  on  weight 
of  hammer,  fall  and  penetration — Formula  suggested  for  bearing  re¬ 
sistance  based  on  bearing  resistance  of  soil  and  frictional  resistance  on 
exposed  surface  of  pile — Only  formula  applicable  to  piles  sunk  by 
water-jet  or  otherwise  forced  in  ground  without  blows  of  hammer — 
Examples  under — Several  forms  of  pile-driver  used— Description  and 
discussion — Hand,  horse,  and  steam-power.  Also  see  Supplement  .  219 

ARTICLE  XLIV. 

Piles: — Purposes  for  which  driven — Long  and  short — Sand  piles — Under 
houses,  piers,  wharves,  and  dikes — Pile-trestles  extensively  used — Dis¬ 
cussions  of  three  and  four  pile  bents,  with  vertical  and  batter  piles — 
Designs  of  floors  or  decks — Economical  considerations — Piles  in  differ¬ 
ent  kinds  of  materials— Piles  on  rock  bottoms — Cribs  often  substi¬ 
tuted  for — Construction  and  sinking  of  cribs . 226 

ARTICLE  XLY. 

Comparative  Estimate  of  Costs  Framed  and  Pile  Trestles: — Tres¬ 
tles  mainly  temporary  expedients,  intended  to  be  replaced  by  iron, 
masonry,  or  earthen  embankments — Relative  cost  and  quantities  in 
framed  and  pile  trestles — Timber  and  iron — Tables  of  iron,  with  drift- 
bolts — Straps — Mortise-and-tenons — Importance  of — Discussion  of 
economic  length  of  span  for  low  and  high  trestles— Calculations  and 
comparisons — Manner  of  estimating  and  paying  for  trestles — Useless 
requirements — Local  customs  important  to  observe — Cutting  piles  off 
under  water — Divers — Cross-cut  and  circular  saws — Structures  resting 
on  piles — How  secured — Proper  alignment  of  piles— Remarks  .  .  235 

ARTICLE  XL VI. 

Embankment  of  Earth  on  Swamps: — Supporting  power  of  swamp  crust 
— Depth  sunk  in  underlying  soft  silt — Logs  and  plank  used  to  support 
— Objections  to  these  methods — General  remarks  on  earth-work — Ma¬ 
terials  for — Form  and  dimensions  of  embankments — Grades— Settle¬ 
ment  of  banks — Borrow-pits — Side  drains  —  Caving  in  of  slopes — 
'Prevention  of — Ballast  for — Cross-ties— Pine,  oak,  and  lignum  vitae — 
Hewn  and  sawn  cross-ties — Costs  of — Recent  methods  of  embanking, 


XVI 


TABLE  OF  CONTENTS. 


PAGE 

as  compared  with  older — Swampy  material  unfit  for  embankments — 
Formula  for  bearing  power  of  soft  materials,  to  be  used  with  caution 
— Soft  stratum  underlying  a  firmer  one,  and  vice  versa  .  .  .  251 


PART  THIRD. 

ARTICLE  XL VII. 

Deep  and  Difficult  Foundations: — Open  crib  and  pneumatic  caisson 
methods — Crib-methods — Discussion  and  examples— General  designs 
for  timber  and  iron  constructions — Methods  of  sinking— Examples — 
Poughkeepsie,  Hawkesbury,  Morgan  City  bridges — Discussion — 
Advantages  and  disadvautages — Difficulties — Costs  and  quantities  .  262 

ARTICLE  XLVIII. 

Pneumatic  Caissons: — Air  an  essential  element — Plenum  and  vacuum 
methods — Uses  of — Principles  and  practical  applications— Working 
chambers — Air-locks — Uses  and  position  of— Shafts,  pipes,  etc. — 
Safety  precautions — Number  of  men  required— Effect  of  com  [tressed 
air  on  men— Selection  of  men — Precautions  for  their  comfort  and 
safety — Paralysis  and  death  in  caisson  work — Means  of  preventing, 
suggested — Signals — Immediate  effect  of  reducing  air  pressure — Ma¬ 
chinery  . 274 


ARTICLE  XLIX. 

Pneumatic  Caissons:— General  designs  and  construction — Examples — 
New  York  and  Brooklyn — Missisippi  at  St.  Louis  and  Memphis — 
Diamond  Shoals  Light-house — Ohio  River  at  Cairo — Susquehanna, 
Schuylkill,  Tombigbee  caissons — Quantities  and  cost — Full  discussion 
of  each  structure— Cribs  and  coffer-dams  on  caissons — Full  details  of 
construction,  sinking,  and  costs— Accidents,  precautions  against — 
Cribs  not  absolutely  necessary — Masonry  may  be  commenced  on  roof 
of  caisson — Coffer-dams  should  always  be  provided — Designs  and 
construction  of  cribs  and  coffer-dams — Uses  and  advantages.  .  .  281? 

ARTICLE  L. 

Caisson  Sinking: — Sand  and  mud  pumps,  and  blowing-out  process  for 
excavating  material — Precautions  necessary  in  removing  material 
from  under  cutting  edges — Some  difficult  cases — Excavating  below 
cutting  edge  in  sand,  clay,  and  silt — Filling  cribs  and  working  cham¬ 
bers  with  concrete — Precautions  in  passing  concrete  through  supply 


TABLE  OF  CONTENTS. 


XVll 


shafts— Mixing  concrete  for— Considerations  requiring  caissons— 
Causes  of  some  accidents— Difficulties  and  costs — Lessons  to  be  learned 
— Discussions  of — Frictioual  resistances  on  outside  surfaces  .  .  302 

ARTICLE  LI. 

Combined  Crib  and  Caisson:  — Design,  construction,  and  uses — First  as  a 
caisson,  second  as  a  crib,  third  as  combined  crib  and  caisson — Con¬ 
sists  of  an  ordinary  caisson  or  open  crib,  provided  with  one  or  more 
removable  roofs,  by  means  of  which  it  can  be  sunk  to  any  desired 
depth  as  a  pneumatic  caisson— One  or  more  roofs  removed,  and  sink¬ 
ing  continued  if  desired  by  open  crib  process — Use  in  small  depths— 
Requiring  only  one  or,  better,  two  roofs — After  sinking,  and  sealing  up 
working  chamber,  roof  removed — Concreting  is  completed  in  open  air 
or  under  moderate  pressure — Better,  more  rapid,  and  satisfactory  work 
— Substituted  for  ordinary  coffer-dams — Construction  and  sinking 
described— Safety  and  comfort  of  men  provided  for — Economy,  cer¬ 
tainty,  and  rapidity  in  sinking — When  sunk  to  depths  of  100  ft.  by 
pneumatic  process,  piles  can  be  introduced  and  driven,  or  sinking 
continued  by  open  crib  method  to  any  greater  depth — Average  lift  of 
dredged  material  decreased,  and  also  cost,  as  compared  with  open  crib 
process — Constructed  of  timber  or  iron,  or  both  combined,  in  any  of 
designs  already  given — General  remarks  on — Unnecessarily  sized  and 
shaped  parts — Cost  and  increased  difficulties  in  construction  of  caissons 
— Poor  designs,  etc . .  311 


ARTICLE  LII. 

All-Iron  Piers: — Of  wrought-iron  columns  resting  on  masonry  piers 
or  pedestals — Description — Advantages,  dangers,  risks — Precautions 
necessary — Screw-pile  piers — Full  description  and  discussion  of 
designs — Methods  of  construction — Sinking  piles — Both  by  turning 
and  by  water- jet— Advantages  and  disadvantages  ....  321 

ARTICLE  LIII. 

Location  of  Piers:— By  triangulation  and  direct  measurements  with 
tapes  or  wires — Instruments  required — Base  lines— Remarks  on  loca¬ 
ting  bridge  sites — Reasons  controlling  same — Examples  of  some  of 
the  longest  spans  and  highest  piers . 325 

ARTICLE  LIV. 

Poetsch  Freezing  Process: — Details  and  discussions — Description  of 
method — Considered  as  the  best  method  of  sinking  through  quicksand.  338 


XVI  1( 


TABLE  OF  CONI  ENTS, 


ARTICLE  LV. 

PAGE 

Quicksand  defined — The  most  difficult  material  to  deal  with  in  putting 
in  foundations — Old  methods  of  sinking  through — Freezing  process 
applicable — A  more  recent  method — by  injecting  cement  grout  under 
pressure  through  pipes  into  the  quicksand,  which  on  setting  converts 
quicksand  into  an  artificial  stone— Discussion  and  description  of  this 
last  method— Sinking  hollow  cylinders  of  brick,  concrete,  or  iron 
through — Methods  and  examples . .  336 

ARTICLE  LVI. 

Foundations  foe  High  Buildings: — Unit  weights  allowed  on  sand,  clay, 
aud  silt — Usual  methods  on  sand  and  clay — Sinking  through  soft 
materials,  by  shafts,  cylinders,  etc.,  to  rock,  or  by  driving  piles — The 
three  methods  compared  and  discussed— Economy  controlling  factor — 
Masonry  on  concrete — Iron  rails  or  beams  imbedded  in  concrete — 
Formulae  for  projection  of  successive  courses — Timber  platforms  or 
grillages  on  natural  material  or  beds  of  concrete — Some  examples  of 
actual  loads — East  River  Bridge — Capitol  building  at  Albany— Bridge 
at  London — Washington  Monument — Tay  Bridge,  Scotland — Hudson 
River  Tunnel— Eiffel  Tower,  Paris— City  Hall,  Kansas  City— Audito¬ 
rium  Building,  Chicago — Bearing  resistance  under  piers  and  frictional 
resistance  on  surface  of  caissons  as  given  usually  uncertain  and 
unreliable— Examples  of— Cairo,  Bismarck,  Susquehanna  rivet- 
bridges — Methods  of  determination  different— Importance  of  accurate 
determinations  and  full  records— Effects  of  compressed  air  in  caissons 
and  escaping  under  cutting  edge  reducing  frictional  resistance— Resist¬ 
ance  to  pulling  piles  less  than  that  to  force  them  down,  with  reasons 
for  same— Examples  of  frictional  resistances— Records  few  and  uncer¬ 
tain  . 343 


ARTICLE  LVII. 

High  Buildings:— General  discussions  of  methods  offered  to  builders— 
First,  direct  building  on  ordinary  soils— second,  timber  platforms, 
grillages,  or  cribs  on  soils— Third,  iron  or  timber  beams  imbedded 
in  concrete — Fourth,  piles  driven  to  rock,  or  supported  by  direct  re¬ 
sistance  at  point  and  by  frictional  resistance  on  surfaces  in  contact 
with  soil— Fifth,  well  sinking,  with  timber-lined  shafts,  brick,  con¬ 
crete,  or  iron  liued  cylinders,  or  by  open  cribs  or  pneumatic  caissons— 
Remarks  on  and  general  discussion  of  methods— Kansas  City  Hall— 
Manhattan  Building,  New  York— Masonry-lined  cylinders— Madras 
Railway,  India — Kentucky  aud  Indiana  Bridge  piers,  Ohio  River 
Iron  cylinders,  brick  and  concrete  lined — Hollow  spaces  filled  with  con¬ 
crete  after  sinking— Methods  and  costs  of  construction.  Also  see  Sup¬ 
plement 


TABLE  OF  CONTENTS. 


XIX 


SUPPLEMENT. 

PAGE 

Hawarden  Bridge — Large  cylinders,  partly  of  iron  and  partly  of  brick — 
Filled  with  concrete — Construction  and  methods  of  sinking — Piles 

sunk  by  water- jet — Description  and  cost . 361t 

Foundations  and  floors  for  the  buildings  of  the  World’s  Columbian  Ex¬ 
position— -Lay  and  character  of  underlying  strata — Load  allowed  per 
square  foot  on  sand,  amount  of  settling — Platforms  and  piles,  when 
used — Pneumatic  work  under  pressure  greater  than  ever  used  hereto¬ 
fore— Tunnel  under  river — Progress,  depths,  etc. — Paralysis  and 
deaths — Compare  favorably  with  preceding  pneumatic  work  .  .  372 

Importance  of  borings  and  soundings,  as  illustrated  by  failure  and  neces¬ 
sary  removal  of  large  pier  constructed  in  Coosa  River,  at  Gadsdeu, 

Ala. — Causes,  consequent  cost,  etc . 373 

Bearing  power  of  piles — Discussion  of  formulte,  with  numerous  records 
of  actual  loads  on  piles  aud  calculated  safe  loads  for  same  by  formula 
— Weights  of  hammers — Falls — Penetration  in  sand,  gravel,  clay,  and 
silt — Numbers — Lengths  aud  general  conditions  of  driving  .  .  376 

Preservation  of  timber  by  vulcanizing  process— Description  of  method, 
pressure,  and  temperature  required — Chemical  changes  and  reactions 
in  fluid  constituents  of  timber — Resulting  product — Experiments 
showing  increased  strength,  stiffness,  and  durability — Tensile  strength 
of  cements — Tests  made  from  1  day  to  4  years — Natural  Portland 
cement . 386 


i 

i 


) 


A  PRACTICAL  TREATISE  ON 
FOUNDATIONS. 


PART  FIRST. 


Article  I. 

FOUNDATION-BED. 

1.  Notwithstanding  the  almost  infinite  variety  of  materials 
upon  which  we  have  to  build  and  do  build,  there  are  certain 
general  principles  that  should  be  followed,  and  which  are 
applicable  in  all  cases. 

2.  First.  The  surface  of  the  foundation-bed,  excepting 
where  piles  are  used,  should  be  perpendicular  to  the  direction 
of  the  resultant  pressure,  i.e.,  horizontal  in  case  of  ordinary 
bridge  piers,  walls  of  houses,  and  in  general,  in  all  cases  where 
the  resultant  pressure  is  vertical ;  and  in  fact  in  cases  where  the 
resultant  pressure  is  inclined  to  the  vertical — as  in  case  of  re- 
taining-walls,  a  horizontal  foundation-bed  will  usually  prove 
to  be  safe.  This  does  not  mean  that  on  solid  rock  the  founda¬ 
tion-bed  must  be  cut  over  its  entire  surface  to  one  horizontal 
surface,  or  even  cut  into  a  series  of  horizontal  surfaces  resem¬ 
bling  steps, — this  costs  a  great  deal  of  time  and  money, — but 
that  the  surface  of  the  foundation  shall  be  so  roughened  as  to 


2 


A  PRACTICAL  TREATISE  OIL  FOUNDATIONS. 


prevent  the  possibility  of  the  substructure  slipping  on  the 
foundation-bed.  Illustrated  by  the  following  diagrams  : 


Fig.  i. — Longitudinal  Section  op  Foundation-bed  on  Rock. 


This  is  especially  applicable  to  a  foundation-bed  of  rock. 
In  all  other  materials  a  uniform  horizontal  surface  or  a  series 
of  steps  will  be  found  both  convenient  and  economical.  And 
in  fact  in  rock  a  series  of  blast  over  the  surface,  making  a 
number  of  irregular  depressions,  will  satisfy  all  conditions  of 
safety. 

3.  Second.  An  excavation  must  be  made  for  a  certain 
depth,  depending  mainly  upon  the  depth  to  which  alternate 
freezing  and  thawing  takes  place;  this  depth — say  from  (2)  two 
to  (6)  six  feet — depending  upon  the  climate  and  latitude,  but 
may  be  limited  in  rock  to  removing  loose  and  disintegrated 
portions. 

4.  Third.  As  far  as  possible,  surface  water  should  be  ex¬ 
cluded  from  the  foundation-bed,  and  all  possibility  of  running 
water  should  be  absolutely  excluded.  This  is  accomplished 
by  surface  drains,  and  where  necessary  by  subsoil  drains. 

The  principles  above  stated  are  applicable  to  rock,  clay, 
sand,  gravel,  and  various  combinations  of  the  three  latter.  . 

5.  Fourth.  Uniformity  of  material  in  the  foundation-bed 
is  absolutely  necessary.  It  is  almost  certain  that  any  kind  of 
material,  except  rock,  will  settle  more  or  less  under  pressure, 
and  will  settle  irregularly,  consequently  the  structure  will 
inevitably  crack  somewhere.  Build  wholly  on  one  or  other  of 
the  materials  mentioned. 

6.  Fifth.  The  weight  of  the  structure  should  be  as  uniform 
as  possible,  and  the  structure  should  be  built  on  all  sides  as 


FO  UN  DA  TION-BED. 


3 


nearly  of  the  same  height  as  possible.  If  heavy  towers,  such 
as  the  spires  of  churches,  and  they  are  bonded  at  all  to  the  body 
of  the  building,  special  provisions  (hereafter  described)  should 
be  made  so  as  to  make  the  unit  pressure  (pressure  per  square 
foot  of  foundation-bed)  the  same  as  under  any  other  part 
of  the  structure. 

7.  The  above  principles  being  followed,  safety  against  slip¬ 
ping  is  fully  provided,  and  partly  against  settling.  But  an¬ 
other  important  element  is  the  unit  weight  or  pressure  per 
square  foot  of  structure  upon  the  foundation-bed.  Our  knowl¬ 
edge  as  regards  the  capacity  of  bearing  weight  is  meagre,  and 
such  as  we  have  is  conflicting  and  uncertain.  The  test  of  a 
cubical  block  of  stone  2  in.  X  2  in.  X  2  in.  of  4  sq.  in.  of  sur¬ 
face,  with  cushions  of  pine,  lead,  or  other  substance,  under 
pressure,  can  scarcely  be  considered  as  determining  the  crush¬ 
ing  resistance  of  immense  volumes  of  the  same  material  in 
quarries  or  when  built  into  massive  structures,  as  valuable  as 
it  may  be  in  other  respects.  But  even  the  strength  thus 
attained  is  sufficient  to  carry  any  load  liable  to  occur  in  prac¬ 
tice. 

8.  To  illustrate:  the  most  reliable  authorities  give  the  resist¬ 
ance  to  crushing  of  weak  sandstone  3000  lbs.  per  square  inch  ; 
a  granite  pier  180  ft.  high,  carrying  one  half  of  two  spans  of 
525  ft.  length  and  a  rolling  load  of  3000  lbs.  per  lineal  foot, 
gives  a  resultant  weight  of  only  150  lbs.  per  square  inch,  giving 
a  factor-of-safety  of  20.  Therefore  we  may  conclude  that 
almost  any  structure  that  we  are  likely  to  build  can  be  safely 
constructed  on  the  three  types  of  rock  commonly  met  with- — 
granite,  limestone,  sandsto n e . 

9.  Some  authorities  class  clay,  sand,  sand  and  gravel  to¬ 
gether,  and  state  that  3000  lbs.  per  square  foot  of  foundation-bed 
is  the  greatest  intensity  of  pressure  admissible.  The  writer, 
however,  gives  to  clay  the  precedence,  for  the  following  rea¬ 
sons  :  Clay  is  more  compact ;  .along  with  its  tendency  to  retain 
water  it  has  an  equal  power  of  excluding  water;  if  settlement 
takes  place  it  is  apt  to  be  uniform  under  same  pressure,  and 
.consequently  is  less  liable  to  cause  damage  to  structure  above  ; 


4 


A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


it  does  not  scour,  and  the  weight  on  such  material  has  a  ten¬ 
dency  to  aid  in  keeping  water  out  of  that  space  over  which 
weight  is  distributed.  Water  can  be  more  easily  kept  from  the 
foundation-bed  either  by  surface  or  subsoil  drains.  The  above 
authorities  do  not  state  the  exact  quality  of  material  alluded 
to,  as  clay  may  vary  from  a  soft,  pliable  clay,  through  loam,  a 
mechanical  mixture  of  clay  and  sand  or  what  might  be  called 
“  brick  clay,”  and  marl,  a  mechanical  mixture  of  carbonate  of 
lime  and  clay,  together  with  certain  silicates  and  protoxide  of 
iron,  then  culminating  in  what  may  be  called  an  indurated  clay. 

10.  If  the  low  unit  pressure,  3000  lbs.,  is  the  limit  of  safety, 
but  relatively  small  structures  should  be  built  upon  it  without 
taking  unusual  precaution  to  distribute  the  pressure  over  a 
large  area  or  by  compacting  the  material  by  the  use  of  piles. 
Taking  the  weight  of  a  brick  wall  at  120  lbs.  per  cubic  foot,  the 
material  above  mentioned  would  only  bear  a  column  of  ma¬ 
sonry  25  ft.  high  and  1  sq.  ft.  base;  but  this  weight  or  pressure 
can  be  easily  distributed  over  two,  three,  or  more  square  feet  of 
foundation-bed.  So  for  a.ny  ordinary  structure  the  above  limit 
of  resistance  need  not  be  exceeded,  and  in  view  of  the  fact  that 
so  many  structures  do  settle  and  often  cause  dangerous  cracks, 
it  is  unwise  to  take  any  risk.  The  writer  built  a  bridge  across 
the  Ohio  River  at  Point  Pleasant,  W.  Va.,  on  what  he  has 
classed  an  indurated  clay — evidently  a  clay  containing  car¬ 
bonate  of  lime.  It  could  be  worked  into  a  paste  with  water. 
Frequently  the  pit  would  be  flooded.  After  pumping  out  the 
water  a  thin  layer  of  slush  or  paste  would  be  found.  When 
this  was  scraped  off,  to  the  depth  of  an  inch,  rarely  more,  the 
surface  was  as  dry  and  as  hard  as  before.  The  largest  pier  was 
about  100  ft.  high,  carrying  one  span  of  400  ft.  and  another  of 
200  ft.,  built  of  sandstone,  producing  approximately  a  pressure 
of  5000  lbs.  per  square  foot  of  foundation-bed,  assuming  sand¬ 
stone  at  150  lbs.  per  cubic  foot  and  spread  doubling  area  of 
base.  These,  then,  can  be  taken  as  the  safe  limits  for  a  clay 
foundation.  Some  clays  have  seams  in  them,  generally  sloping 
at  a  greater  or  less  angle  to  the  vertical :  these,  if  extensive,  are 
dangerous,  as  the  water  will  percolate  along  them,  causing  a 


FOUNDA  TION-BED. 


5 


dangerous  tendency  to  slide.  In  these  cases  the  water  must  be 
excluded  or  the  depth  cut  into  material  greatly  increased. 

11.  Building  on  sand  was  pronounced  dangerous  in  the 
Bible,  and  has  been  so  considered  ever  since ;  but  circum¬ 
stances  often  compel  us  to  build  on  this  tempting  material, 
and  as  it  may  be  said  take  the  chances.  Sand,  when  confined,  is 
considered  practically  incompressible  within  the  limit  of  actual 
crushing  the  grains  of  sand  into  impalpable  powder.  Sand  will 
hold  your  structures  if  you  can  hold  the  sand.  But  here  is  the 
difficulty:  it  is  porous,  and  unless  confined  in  walls  of  rock  or 
clay  there  is  always  danger  of  the  water  passing  through, 
scouring  out  the  material,  and  undermining  the  foundation, 
this  process  being  greatly  aided  by  the  weight  of  the  struc¬ 
ture,  and  sometimes  forming  with  water  quicksand,  which  is 
almost  as  unstable  as  water  itself.  Therefore  in  building  on 
sand  under  no  circumstances  exceed  the  limit  of  weight  of  gooo 
lbs,  per  'square  foot  of  foundation-bed,  and  in  addition  be  sure 
of  excluding  the  water,  or  in  exposed  situations  drive  piles. 
More  on  this  point  hereafter.  Beds  of  gravel  and  bowlders 
especially  can  certainly  be  relied  upon,  to  at  least  the  superior 
limit  for  clay  of  5000  lbs.  per  square  foot  of  bearing  surface. 
Two  of  the  high  piers  of  the  Susquehanna  River  bridge, 
B.  &  O.  R.  R.  at  Havre  de  Grace  were  built  on  bowlders 
large  and  small,  but  at  a  great  depth  below  the  bed  of  the 
river,  in  which  the  frictions  on  the  sides  supports  much  of  the 
weight. 

12.  The  remaining  material  of  silt  or  slush,  such  as  we 
find  in  all  the  swamps,  especially  in  the  Southern  States,  can 
scarcely  be  made  safe  without  the  use  of  piles,  for  very  heavy 
structures;  but  by  the  liberal  use  of  broken  stone,  or  even  in 
some  cases  of  sand  or  gravel,  a  reasonably  stable  foundation- 
bed  may  be  artificially  constructed,  which  will  be  fully 
explained  further  on.  The  above  sets  forth  fully  the  actual 
and  relative  merits  of  foundation-beds  generally  met  with  in 
actual  practice.  All  of  these  matters  will  be  incidentally 
alluded  to  when  we  come  to  discuss  foundations,  which  is  the 
next  division  of  the  subject  to  be  treated. 


6 


A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


13.  A  combination  of  these  materials  is  frequently  met 
with,  the  bearing-power  of  which  may  practically  be  taken  the 
same  as  above;  but  frequently  these  combinations  take  the 
form  of  what  is  called  hardpan,  or  cemented  sand  and  gravel, 
that  would  certainly  justify  a  higher  classification,  and  would 
not  be  inferior  to  the  ordinary  kinds  of  rock.  There  is  no  mis¬ 
taking  this  material  when  met  with,  and  it  can  be  relied  upon 
to  bear  the  weight  safely  of  any  ordinary  structure.  In  many 
of  the  Southern  States  there  is  an  earthy  substance  which 
may  be  called  a  marl,  easily  cut  into  blocks,  difficult  to  exca¬ 
vate,  requiring  blasting  sometimes,  and  capable  of  bearing 
heavy  loads,  but  disintegrating  rapidly  when  exposed  to  the 
air,  and  consequently  unfit  for  building  purposes.  The  writer 
founded  several  piers  on  this  material,  carrying  long  spans  275 
ft.  in  length  ;  the  piers  were  of  brick  and  the  pneumatic  cais¬ 
son  used,  passing  through  sand  and  silt  before  reaching  it. 
This  material  almost  disintegrated  by  slacking  when  exposed  ; 
it  effervesced  freely  with  acids. 

14.  The  practical  deductions  from  the  above,  then,  may  be 
stated  as  follows  : 

1st.  That  it  is  in  general  perfectly  safe  to  build,  on  any 
material  that  can  be  called  rock,  any  structure  likely  to  be 
required. 

2d.  Bowlders  and  gravel  can  also  be  considered  perfectly 
reliable  for  any  ordinary  structure  undpr  any  ordinary  condi¬ 
tions.  Scour  alone  should  be  guarded  against,  which,  however,, 
is  not  probable. 

3d.  Sand  is  safe  to  bear  a  load  of  any  amount,  provided  it 
is  confined  ;  but  great  precaution  must  be  taken  to  confine  it, 
and  also  keep  water,  especially  running  water,  from  it. 

4th.  Clay,  when  compact  and  dry,  will  likewise  carry  very 
large  loads.  Water  should  be  kept  from  it  both  under  and 
around  the  structure,  as  it  may  give  way  if  it  gets  in  the  con¬ 
dition  of  paste  by  bulging  up  around  the  structure. 

5th.  In  the  last  three  cases  the  base  of  the  structure  should 
be  so  spread  out  as  to  keep  the  pressure  per  square  foot  of 
base  within  the  safe  limit,  and  the  depth  below  the  surface 


FOUNDATION-BED.  7 

must  be  below  the  action  of  frost,  which  varies  from  2  ft.  to 
6  ft.;  and  in  soft  kinds  of  material  the  deeper  the  better. 

15.  A  thick,  hard,  or  compact  strata  overlying  a  much  softer 
one,  even  silt  or  quicksand,  will  often  carry  a  considerable  load, 
the  hard  strata  as  it  were  floating  on  the  softer.  It  is  some¬ 
times  better  not  to  break  through  it,  as  it  has  the  effect  of 
spreading  the  base  and  distributing  the  pressure  over  a  large 
area.  Good  judgment  is  here  required,  and  some  risk  must  be 
run.  This  principle  is  followed  when  planks  or  logs  are  spread 
out  on  the  soft  material,  and  the  structure  built  on  the  logs, 
the  logs  forming  a  broad  bearing  surface.  Mr.  Rankine  states 
that  Chat  Moss  was  crossed  by  the  use  of  dry  peat  and  hurdles 
or  fascines  in  layers  forming  a  raft,  which  carried  a  railway  on 
it.  It  would  seem  safer  and  more  satisfactory  in  such  cases  to 
drive  piles. 

16.  The  following  figures  give  the  actual  bearing-power  of 
some  of  the  above  materials. 

Mr.  Rankine  says,  page  361,  “Civil  Engineering:” 

Granite .  12,861  lbs.  per  square  inch. 

Sandstone .  9,842  “  “  “  “ 

Soft  sandstone.  ..  3,000  to  3,500  “  “  “  “ 

Strong  limestone .  8,528  “  “  “  “ 

Weak  limestone .  3,050  “  “  “  “ 

Clay,  sand,  and  gravel. .  17  to  23  “  “  “  “ 

Brick .  1,100  “  “  “  “ 

And  gives  the  actual  pressure  on  some  existing  foundations 
as  only  about  140  lbs.  to  the  square  inch,  giving  an  actual 
factor-of-safety  of  about  22,  whereas  factor-of-safety  from  8 
to  10  is  considered  ample.  These  are  probable  average 
values. 

i7-  Mr.  Baker,  in  his  treatise  on  Masonry  Construction, 
page  10,  gives  the  following  as  the  crushing  strength  of  stone  : 

Granite  from  12,000  to  21,000  lbs.  per  sq.  in.  =  860  to  1,510  tons  per  sq.  ft. 

Marble  from  8,000  to  20,000  lbs.  per  sq.  in.  =  580  to  1,440  tons  per  sq.  ft. 

Limestone  from  7,000  to  20,000  lbs.  per  sq.  in.  =  500  to  1,440  tons  per  sq.  ft. 


3 


A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


Sandstone  from  5,000  to  15,000  lbs.  per  sq.  in.  =  360  to  1,080  tons  per  sq.  ft. 
Brick  from  674  to  13,085  lbs.  per  sq.  in.  =  48  to  936  tons  per  sq.  ft. 

Clay  from  28  to  84  lbs.  per  sq.  in.  =  2  to  6  tons  per  sq.  ft. 

Gravel  from  112  to  1,401  lbs.  per  sq.  in.  =  8  to  10  tons  per  sq.  ft. 

The  above  doubtless  gives  the  results  of  the  latest  experi¬ 
ments.  There  are  special  cases  when  the  loads  actually  borne 
are  greater  than  the  above  ;  but  we  can  safely  conclude  that 
good  ordinary  clay  will  carry  safely  two  tons  per  square  footj_ 
sand,  from  3  to  4  tons  to  the  square  foot,  provided  it  can  be 
kept  entirely  free  from  water. 

18.  In  cases  of  doubt  and  the  absence  of  precedent,  when 
unusually  heavy  loads  are  to  be  carried,  and  especially  when 
the  weight  of  the  structure  is  not  uniformly  distributed,  as  in 
case  of  high  towers  and  spires,  tests  should  be  made  by  actual 
weights  placed  on  a  unit  of  area,  which  can  be  done  at  the  cost 
of  but  little  time  and  money  ;  and  as  the  means  are  always  in 
reach  to  make  the  foundation  safe,  it  is  certainly  inexcusable, 
to  say  the  least  of  it,  to  blunder  along  and  take  the  chances  of 
the  structure  falling,  involving  great  loss  of  property,  if  not  of 
life,  and  only  to  avoid  expending  a  few  dollars. 

19.  When  structures  fail,  it  may  in  general  be  said  that  it 
is  impossible  to  determine  the  cause,  though  in  general  it  is 
easy  to  get  numberless  opinions  of  so-called  experts,  and  with 
these  the  public  and  juries  are  satisfied  ;  but  in  a  large  majority 
of  cases  it  can  be  traced  to  that  part  of  the  structure  under 
ground  or  under  water,  and  ultimately  due  to  the  failure  of  the 
foundation-bed  :  for  even  if  the  part  of  the  structure  under 
ground  is  defective  in  some  of  its  parts,  it  throws  an  excessive 
weight  on  some  part  of  the  foundation-bed.  The  failure,  from 
high  winds,  from  thrusts  of  roof  or  floors,  or  from  floods,  drift, 
and  ice,  is  generally  indicated  by  the  manner  of  the  falling  ; 
and  though  this  may  evidently  be  the  direct  cause  of  failure, 
yet,  indirectly  the  foundation-bed  is  at  fault,  as  these  cause 
undue  pressure  on  some  parts  or  scour  out  the  foundation- 
beds  and  undermine  the  structure,  as  other  conditions  and  re¬ 
quirements  always  require  such  weights  and  sizes  of  structures 
as  will  resist  the  outside  forces.  The  dimensions  of  bridge 


FOUND  A  TIONS. 


9 


piers  are  regulated  generally  by  the  dimensions  at  the  top  re¬ 
quired  as  a  rest  for  the  bridge  structure,  and  are  greater  than 
that  necessary  to  withstand  the  effects  of  these  external  forces. 
He  sure  of  your  foundation  and  foundation-beds,  and  except  in 
extreme  cases  the  upper  part  of  the  structure  will  take  care  of 
itself. 


Article  II. 

FOUNDATIONS. 

20.  THIS  division  of  the  subject  includes  that  part  of  the 
substructure  reaching  from  the  foundation-bed  to  the  surface 
of  the  ground  or  the  surface  of  the  water,  and  necessarily  in¬ 
cludes  the  various  means  of  reaching  the  foundation-bed,  such 
as  ordinary  excavations  on  land,  driving  piles  on  land  or  in 
water,  screw-pile  foundations,  Cushing  cylinders,  coffer-dams, 
pneumatic  cylinders,  pneumatic  caissons,  open  caissons,  pierre- 
perdue  foundations  on  land  or  in  water,  sand  foundations  in 
swamps,  concrete  foundations,  rubble-stone  foundations,  etc. 

21.  Each  of  these  divisions  will  be  treated  more  or  less 
elaborately,  but  purely  from  a  practical  standpoint  and  as  con¬ 
cisely  as  the  importance  of  the  subject  may  demand,  consist¬ 
ently  with  that  amount  of  detail  as  may  be  necessary  to  a  clear 
understanding  of  the  matter.  These  will  also  be  accompanied 
by  drawings  giving  sufficient  details  to  be  of  actual  and  practi¬ 
cal  use.  Many  books  mystify  with  useless  formulae,  and  give 
just  enough  practical  information  and  details  as  to  leave  you 
in  doubt  whether  you  know  anything  at  all,  as  it  is  generally 
admitted  that  in  many  cases  the  formulae  have  no  practical 
value.  This  the  writer  hopes  to  avoid,  and  at  the  same  time 
not  to  extend  the  limits  of  this  subject  too  far. 

CONCRETE. 

22.  As  concrete  is  used  so  extensively,  and  in  combination 
with  almost  all  kinds  of  foundations,  we  will  commence  with 
this  material. 


IO 


A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


Concrete  is  composed  of  broken  stone  or  gravel  or  both, 
sand,  cement,  and  water,  mixed  under  certain  circumstances  in 
absolutely  definite  proportions,  so  as  to  obtain  a  conglomera¬ 
tion  which  experiments,  conducted  principally  by  Government 
engineers,  have  shown  ultimately  to  produce  the  best  possible 
results  ;  and  doubtless  in  all  works  this  practice  would  be  fol¬ 
lowed,  if  in  works  paid  for  by  private  individuals  or  companies, 
we  had  the  money  and  time  at  our  disposal.  But  in  works  of 
this  class  we  must  aim  to  attain  as  near  perfection  as  practica¬ 
ble,  but  be  satisfied  with  what  is  good  enough  for  the  purpose 
in  view,  with  the  least  possible  cost  in  time  and  money,  con¬ 
sistent  with  securing  a  permanent,  strong,  safe,  and  durable 
structure.  We  will  first,  however,  explain  the  process  of  mak¬ 
ing  concrete  in  accordance  with  the  requirements  of  the  Gov¬ 
ernment  engineers. 

23.  Gen.  0.  A.  Gillmore’s  treatise  on  Limes,  Hydraulic 
Cements,  and  Mortars  is  assumed  to  be  high  authority,— a  book 
which  contains  valuable  and  interesting  information.  On  page 
226,  paragraph  450,  we  find  :  “  The  concrete  was  prepared  by 
first  spreading  out  the  gravel  on  a  platform  of  rough  boards, 
in  a  layer  from  eight  to  twelve  inches  thick,  the  smaller  peb¬ 
bles  at  the  bottom  and  the  larger  on  top,  and  afterwards 
spreading  the  mortar  over  it  as  uniformly  as  possible.  The 
materials  were  then  mixed  by  four  men,  two  with  shovels  and 
two  with  hoes ;  the  former  facing  each  other  and  always 
working  from  the  outside  to  the  centre,  then  stepping  back 
and  recommencing  in  the  same  way,  and  thus  continuing  the 
operation  until  the  whole  mass  was  turned.  The  men  with' 
hoes  worked  each  in  conjunction  with  a  shoveller,  and  were 
required  to  rub  well  into  the  mortar  each  shovelful  as  it  was 
turned  and  spread,  or  rather  scattered  on  the  platform  by  a 
jerking  motion.  The  heap  was  turned  over  a  second  time  in 
the  same  way,  but  in  the  opposite  direction  ;  and  the  ingredi¬ 
ents  were  thus  thoroughly  incorporated,  the  surface  of  every 
pebble  being  well  covered  with  mortar.  Two  turnings  usually 
sufficed  to  make  the  mixture  complete,  and  the  resulting  mass 
of  concrete  was  ready  for  transportation  to  the  foundation. ,v 


FOUND  A  TIONS. 


I  I 

There  is  but  little  comment  to  make ;  the  method  for  hand¬ 
mixing  can  be  safely  recommended.  The  writer  has  mixed  large 
quantities  in  practically  the  same  manner,  with  these  modi¬ 
fications:  Firstly,  the  broken  stone  or  gravel  was  not  screened 
so  as  to  separate  the  larger  from  the  smaller  sizes,  and  place 
the  smaller  pebbles  at  the  bottom  and  the  larger  on  top.  The 
broken  stone  or  gravel,  within  special  limits  as  to  the  large 
size,  the  limit  being  such  as  would  pass  through  a  2^-inch  ring, 
determined  by  inspection,  and  used  the  material  as  delivered ; 
and  secondly,  that  no  hoes  were  used,  all  the  men  using  the 
shovel  as  described,  and  each  shoveller  as  he  turned  over  his 
shovelful  made  three  or  four  cuts  into  the  mass  with  his 
shovel  in  a  nearly  vertical  position,  the  object  being  to  ram 
the  mortar  between  and  over  the  broken  stone,  and  also  pre¬ 
vent  the  mass  from  being  heaped  up,  which  would  cause  the 
stone  to  roll  down  to  the  base  of  the  mass,  and  leaving  a  sur¬ 
plus  of  mortar  on  top.  This  operation  was  continued  until 
every  stone  was  covered.  Mixing  by  hand  is  rarely  economi¬ 
cal  or  rapid  enough  where  large  quantities  of  concrete  are  to 
be  made  in  a  limited  time.  The  method  of  mixing  mortar, 
together  with  the  ingredients  and  proportions  of  the  same, 
whether  mixed  by  hand  or  machinery,  are  elaborately  explained 
in  Gen.  Gillmore’s  treatise,  pages  192  to  206  inclusive,  to 
which  for  valuable  information  the  reader  is  referred. 

24.  The  consistency  of  the  mortar — whether  very  soft,  in  a 
pasty  condition,  or  almost  dry — is  not  explained.  This  is  an 
important  consideration,  and  one  upon  which  there  is  a  wide 
difference  of  opinion.  In  the  Appendix  to  Gen.  Gillmore’s 
treatise  he  speaks  of  the  mortar  as  being  “  about  the  consistency 
of  plasterer’s  mortar.”  In  an  extended  experience  the  writer 
of  this  work  has  found  this  consistency  to  give  the  best  results 
in  many  ways,  can  be  more  readily  incorporated,  as  well  as 
more  uniformly  mixed;  can  be  handled  more  readily;  can  be 
compacted  by  ramming  without  producing  a  spongy,  springy 
mass;  takes  its  initial  set  more  readily;  and  certainly  for  ordi¬ 
nary  purposes  is  more  satisfactory  than  when  the  mortar  is 
more  liquid,  as  well  as  when  it  is  too  dry  and  stiff.  In  the  Ap- 


12 


A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


pendix  Gen.  Gillmore  gives  some  valuable  information  on  the 
cost,  qualities,  and  proportions  of  ingredients  of  concrete  on 
Staten  Island,  which  is  well  worth  studying,  as  well  as  methods 
of  mixing  mortar  and  concrete  by  hand  and  machinery.  Only 
one  or  two  tables  of  proportions  will  be  given. 

“  Concrete  No.  I  : 

i  bbl.  German  Portland  cement,  )  5.4  bbls.  concrete 

5^  “  damp  sand  loosely  measured,  )  mortar. 

6  “  gravel  and  pebbles  from  sea-shore, 


9  “  broken  stone, 


Producing  50  feet  of  rammed  concrete.  This  concrete  is  of 
first-rate  quality,  being  compact,  free  from  voids,  and  strong. 
It  is  richer  in  mortar  than  would  be  necessary  for  most  pur¬ 
poses.”  Proportions  1  mortar  to  2^  stone  and  gravel — evidently 
a  large  excess  of  mortar  over  quantity  necessary  to  fill  voids. 

In  the  writer’s  experience  2  barrels  of  sand  to  1  barrel  of 

cement  for  ordinary  and  3  barrels  of  sand  to  1  barrel  of  Port¬ 
land  cement  seem  to  be  the  rule  for  use  in  constructing  foun¬ 
dations  for  bridges  of  great  magnitude.  For  less  important 
work  from  4  to  5  of  sand  to  one  of  cement. 

“  Concrete  No.  5  : 

1  bbl.  Rosendale  cement,  \ 

3  “  damp  loose  sand,  -  3.27  bbls.  concrete  mortar. 

5  “  broken  stone,  ) 

Will  yield  21.75  cu-  ft.,  rammed  in  position.”  This  mortar 
possesses  a  crushing  strength  of  130  lbs.  per  square  inch  when 
two  months  old.  “  Another  proportion  given  : 

4  barrows  of  mortar  (8  cu.  ft.); 

6  heaped-up  barrows  of  broken  stone  (14  cu.  ft.); 

6  heaped-up  barrows  of  gravel  (14  cu.  ft.).” 

This  would  seem  a  good  proportion  of  ingredients.  No  mention 
is  made  of  the  resulting  quality  of  concrete. 

25.  It  will  be  observed  from  the  proportions  above  given 


FO  UN  DA  TIONS. 


13 


that  Government  engineers  seem  to  prefer  an  admixture  of 
gravel  with  the  broken  stone,— presumably  to  save  mortar.  It 
is  rarely  the  case  that  gravel  and  stone  can  be  economically 
secured  at  the  same  time,  and  consequently  as  a  rule  only  one 
of  these  elements  can  be  used  ;  and  when  it  can  be  done  the 
chances  are  that  one  part  of  the  concrete  will  be  largely  of 
stone  and  the  other  largely  of  gravel,  as  there  is  no  known  law 
by  which  gravel  can  be  forced  to  place  itself  between  the 
pieces  of  stone.  Either  alone  makes  good  concrete,  as  doubt¬ 
less  a  mixture  of  the  two  will ;  but  many  would  prefer  the  an¬ 
gular  and  rough  broken  stone  to  round  and  smooth  gravel, 
provided  the  stone  is  as  hard  as  granite  or  limestone,  or  some 
of  the  varieties  of  hard  sandstone.  Broken  bricks  and  shells 
are  often  used  in  localities  where  neither  stone  nor  gravel  can 
be  found ;  gravel  would  evidently  be  preferable  to  brick  or 
shells.  The  mortar  takes  hold  of  the  broken  stone,  thereby 
tying  and  binding  the  whole  mass  together,  which  does  not 
take  place  when  gravel  is  used,  as  can  easily  be  seen  by  break¬ 
ing  a  block  thus  made  :  the  gravel  pulls  away  from  the  matrix 
or  mortar,  leaving  round,  smooth  holes.  For  most  purposes 
concrete  has  only  to  bear  a  crushing  strain,  and  is  not  sub¬ 
jected  to  a  tensile  strain  unless  a  foundation  is  undermined, 
which  ought  not  to  occur  often. 

26.  The  writer  has  used  over  30,000  cu.  yds.  of  concrete, 
supervising  to  a  considerable  extent  the  mixing,  in  all  its  de¬ 
tails,  personally,  but  owing  to  the  circumstances  under  which 
he  was  placed,  it  was  impossible  to  give  that  particular  atten¬ 
tion  to  exact  proportions  as  might  conduce  to  the  very  best 
results,  but  certainly  good  enough  for  the  purposes  intended, 
as  it  has  stood  for  years  bearing  enormously  heavy  steady 
loads,  and  the  heaviest  known  rolling  loads  running  at  the 
highest  speed  :  therefore  he  can  say  that  he  has  fully  complied 
with  all  the  conditions  of  good  work,  strength,  durability, 
safety,  with  the  least  cost  and  time  ;  and  even  permanency 
can  safely  be  claimed.  He  will  therefore  give  the  benefit  of 
his  experience  on  such  bridges  as  the  Ohio  River  bridge,  the 
Susquehanna  and  Schuylkill  River  bridges  on  the  B.  &  O. 


14  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 

R.  R.,  and  the  Tombigbee  River  bridge  in  Alabama,  each  of 
which  will  present  some  difference. 

27.  Taking  them  in  order.  We  used  concrete  resting  on 
indurated  clay;  there  were  four  piers  resting  on  the  concrete. 
The  mortar  was  2  sand,  1  cement,  composed  of  a  fair  average 
sand,  clean  and  sharp;  the  cement  used  was  known  as  the 
Louisville  cement.  These  were  mixed  by  hand  in  propor¬ 
tions  of  1  cement  and  2  sand,  water  sufficient  to  form  a  paste 
of  the  consistency  of  plasterer’s  mortar;  the  sand  and  cement 
were  thoroughly  mixed  dry  by  turning  over  and  over  with 
shovels.  This  mixture  was  then  formed  into  a  circular  dam, 
and  a  small  quantity  of  water  was  poured  inside ;  a  portion  of 
the  dry  mixture  was  pulled  by  hoes  towards  the  centre  and 
thoroughly  mixed  with  the  water,  care  being  taken  not  to  let 
the  water  escape,  as  it  would  carry  the  cement  off.  If  this 
mixture  was  too  dry,  more  water  was  added  and  thoroughly 
mixed,  and  this  process  continued  until  the  entire  batch  was 
of  the  proper  consistency.  The  broken  stone  was  a  hard 
bluish-gray  sandstone  found  near  by,  and  small  enough  to  pass 
through  a  ring  2  inches  in  diameter — as  close  as  could  be  ex¬ 
pected.  A  thin  layer  of  this  stone  was  spread  on  a  platform, 
upon  this  a  layer  of  mortar,  on  top  of  which  another  layer  of 
stone,  and  then  another  of  mortar;  this  was  then  turned  over 
and  over  with  shovels  as  previously  described,  until  every 
stone  was  coated  with  mortar;  and  it  presented  a  uniform  ap¬ 
pearance  of  mortar  and  stone  mixed.  On  this  work  the  mix¬ 
ing  was  generally  done  in  the  foundation  pit,  and  the  concrete 
was  then  thrown  with  shovels  into  layers  of  about  10  to  12 
inches  thick,  and  rammed  in  place.  A  pine  plank,  3  inches 
thick  by  12  inches  broad,  cut  in  the  form  of  a  rammer,  seemed 
to  serve  the  purpose  better  than  a  round  heavier  rammer, 
suggested  by  ramming  clay  puddle.  The  ramming  was  con¬ 
tinued  until  a  thin  skim  of  water  appeared  on  the  surface, 
then  another  layer  of  concrete  was  put  on  top  of  this.  Under 
some  of  the  piers  clean  river  gravel  was  used  instead  of  broken 
stone,  mixed  and  compacted  in  place  as  above,  with  equally 
satisfactory  results.  The  proportions  were  usually  I  barrow 


FOUND  A  TIONS. 


15 


of  mortar  to  2 \  barrows  of  stone,  varied  somewhat  as  the  size 
of  the  stone  varied.  With  a  little  experience  the  proportions 
would  be  easily  adjusted  by  the  eye,  the  aim  being  to  have  all 
the  interstices  filled.  The  broken  stone  was  moistened.  We 
secured  a  reasonably  uniform  result.  The  quantity  here  was 
not  very  great — 649  cu.  yds. 

28.  At  the  Susquehanna  and  Schuylkill  River  bridges  all 
the  concrete  in  the  cribs  above  the  caisson  roof  was  mixed  by 
machinery.  All  that  portion  of  the  concrete  in  the  working 
chamber  of  the  caisson  was  mixed  by  hand,  as  above  described, 
only  small  quantities  being  used  at  a  time.  The  concrete  for 
the  crib  was  mixed  as  follows  :  A  revolving  drum  with  buckets, 
similar  to  those  on  an  overshot  water-wheel,  proportioned  so 
as  to  carry  2  or  3  of  sand  to  1  of  cement,  fed  through  two 
distinct  hoppers,  dropped,  as  it  revolved,  the  sand  and  cement 
into  a  trough  in  which  was  placed  a  revolving  worm-screw 
about  10  feet  long  ;  the  sand  and  cement  were  carried  around 
and  forward,  thoroughly  mixing  them  dry  ;  at  a  certain  point, 
determined  by  experiment,  water  was  admitted  from  a  spigot ; 
experiment  determined  how  much  was  necessary  to  be  admit¬ 
ted.  Water,  sand,  and  cement  were  now  turned  over  and  car¬ 
ried  forward  ;  everything  was  so  adjusted  that  at  the  end  of 
the  trough  a  paste  of  the  proper  consistency  was  found  (this 
apparatus  was  the  invention  of  Charles  Sooysmith,  one  of  the 
contractors).  At  the  end  of  the  trough  the  mortar  dropped 
into  the  concrete  mixer,  which  can  best  be  described  as  about 
two  thirds  of  an  iron  cylindrical  pug-mill,  6  or  8  feet  long, 
gently  sloping  downwards  from  the  end  of  the  trough,  the 
arms  of  the  revolving  shaft  in  the  mixer  being  so  set  as  to 
cause  the  materials  in  the  mixer  to  be  revolved  over  and  over 
and  at  the  same  time  moved  forward.  The  proper  proportion 
of  the  broken  stone  to  a  barrel  of  cement  having  been  collected 
near  the  upper  end  of  the  mixer,  it  was  shovelled  into  jthe 
mixer  as  the  mortar  dropped  in  from  the  trough.  Intelligent 
men  soon  learned  to  shovel  at  a  uniform  rate,  and  would  com¬ 
monly  throw  in  with  reasonable  approximation  the  proper 
proportion  of  stone  to  mortar  delivered.  The  concrete  by  the 


1 6  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 

time  it  reached  the  lower  end  of  the  mixer  was  thoroughly 
mixed,  and  then  dropped  into  wheelbarrows  and  carried  to 
the  place  of  deposit.  There  were  defects  in  this  method.  Ab¬ 
solute  uniformity  was  not  obtained,  but  even  then  we  had  a 
remedy  :  if  the  concrete  when  it  dropped  into  the  barrows  was 
too  wet  or  too  dry,  or  had  a  larger  proportion  of  stone  than  the 
mortar  could  carry,  or  not  thoroughly  mixed,  it  was  thrown 
away,  and  the  proportions  readjusted.  Some  waste  resulted; 
some  little  time  was  wasted.  The  proportions  aimed  to  be 
used  were  as  i  of  mortar  to  2\  of  broken  stone.  The  concrete 
for  these  structures  was  generally  dropped  from  a  greater  or 
less  height,  as  the  timber  work  was  always  built  well  ahead  of 
the  concrete ;  but  nevertheless  it  was  distributed  in  layers  with 
the  shovel,  and  rammed  as  before  described. 

29.  The  stone  at  the  Susquehanna  was  granite,  at  the 
Schuylkill  limestone,  broken  in  both  cases  by  the  Gates  crusher. 
No  attempt  was  made  to  screen  the  stone  ;  the  impalpable  dust 
to  a  large  extent  was  blown  away;  but  the  stone  as  it  came  from 
the  crusher  was  delivered  at  the  caisson,  and  consisted  of  stones 
say  from  3  inches  in  diameter  through  all  sizes  down  to  the 
size  of  coarse  sand  :  this  was  taken  into  consideration  in  pro¬ 
portioning  the  sand  in  the  mortar.  The  broken  stone  was 
generally  kept  moist,  always  in  hot  weather.  In  the  crib  of  one 
of  the  piers  at  the  Schuylkill,  as  a  matter  of  economy,  the  crib 
was  filled  with  what  may  be  called  rubble-work,  one-man  stones 
being  simply  imbedded  in  mortar.  Great  care  is  necessary  in 
this  kind  of  work  to  secure  a  solid,  compact  structure,  and 
there  is  danger  of  great  waste  of  mortar  ;  but  why  it  should  not 
be  as  good  as  concrete  in  large  masses  is  probably  hard  to 
explain,  as  to  some  extent  it  does  look  like  folly  to  break  stones 
up  simply  to  cement  them  together  again  :  but  good  practice 
does  seem  to  lean  towards  concrete.  At  both  of  these  bridges 
large  stones  (one-man  stone)  were  placed  at  intervals  on  the 
surface  of  a  layer  of  concrete  and  then  covered  over  with  an¬ 
other  layer,  of  which,  however,  the  writer  doubts  the  wisdom. 
It  may  do  no  harm,  but  surely  it  does  no  good  :  it  would  not 
lessen  the  cost  or  the  time.  All  concrete  or  all  rubble  is  best. 


FOUNDATIONS. 


*7 


30.  As  to  the  Tombigbee  River  bridge,  located  in  the  almost 
limitless  swamps  of  Alabama,  there  was  nothing  especially 
notable,  except  its  inaccessibility,  and  the  almost  total  absence 
of  building  material  of  any  kind,  except  we  may  say  good  pine 
timber.  It  is  true  a  limited  amount  of  gravel  could  be  found, 
but  this  mixed  with  sediment  from  the  frequent  overflows. 
Good  sand  could  be  found  in  places ;  the  gravel  had  to  be 
v/ashed.  We  were  compelled  therefore  to  use  broken  brick, 
which  had  to  be  brought  from  Mobile  on  barges  a  distance  of 
over  a  hundred  miles;  a  small  quantity  of  broken  stone  left 
there  by  incoming  vessels,  which  had  been  used  as  ballast ;  con¬ 
sequently  oyster-shells  brought,  by  schooners  hundreds  of  miles 
distant,  from  oyster-banks  had  to  be  relied  upon,  and  this  had 
to  be  provided  and  delivered  at  high  stages  of  the  water.  The 
mixing  was  by  hand  as  previously  described  ;  there  was  nothing 
new  or  novel,  except  materials  used.  These  materials  for  con¬ 
crete  are  the  last  resort  of  engineers,  and  of  the  two  broken 
brick  is  the  best.  But  much  can  be  done  with  good  cement 
and  clean  sharp  sand. 

31.  The  following  general  principles  should  be  observed  in 
making  concrete : 

Use  good  cement  and  clean  sharp  sand  for  the  mortar,  in 
proportions,  depending  upon  the  quality  of  the  cement,  of  2 
to  4  of  sand  to  1  of  cement ;  sufficient  water  to  produce  a  some¬ 
what  soft  and  plastic  paste. 

Use  the  hardest  stone  available,  granite,  limestone,  hard 
varieties  of  sandstone,  gravel,  etc.  This  to  be  broken  as  nearly 
as  practicable  so  as  to  pass  through  a  ring  of  2  inches  in 
diameter.  Moisten  the  stones  certainly  in  hot  weather.  Use 
somewhat  more  mortar  than  is  necessary  to  fill  the  voids,  which 
will  depend  upon  the  size  of  the  stone,  whether  broken  by  hand 
or  machinery,  also  upon  the  material ;  but  in  general  from 
2  to  4  volumes  of  broken  stone  to  1  of  mortar.  Mix  thoroughly 
the  sand  and  cement,  and  mix  thoroughly  the  mortar  and 
stone. 

Deposit  the  concrete  in  layers  of  not  over  12  inches  in  thick¬ 
ness,  and  ram  until  a  thin  skim  of  water  appears  on  the  surface. 


1 8  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 

Mortar  scarcely  moistened  is  recommended  by  some  engi¬ 
neers  as  producing  ultimately  the  best  result. 

31^.  In  a  letter  from  Gen.  T.  L.  Casey,  U.  S.  Engineer,  the 
proportions  of  cement,  sand,  pebbles,  and  broken  stone  for  the 
concrete  sub-foundation  of  the  Washington  Monument  were 
given  as  follows : 

“  1  volume  of  cement,  dry ; 

“2  volumes  of  sand,  clean,  sharp,  and  medium  size; 

“  3  volumes  of  pebbles,  clean,  and  varying  in  size  from  a 
buck-shot  to  pigeon’s  egg  ; 

“4  volumes  of  broken  stone,  clean,  and  small  enough  to 
pass  through  a  2-inch  ring. 

“  A  ‘  batch  ’  consisted  of  f  barrel  of  cement,  1^  barrels  of 
sand,  2\  barrels  of  pebbles,  3  barrels  of  broken  stone.  In  dry 
weather  about  10  gallons  of  water  was  used  to  a  batch,  but  in 
wet,  soaking  weather  no  water  was  added.  The  ingredients  were 
mixed  in  a  cubical  box  4  ft.  on  each  edge,  rotating  on  a  diag¬ 
onal  axis  passing  through  the  box.  The  mixer  was  turned 
eight  times  for  each  batch.  The  concrete  when  emptied  from 
the  box  was  about  as  moist  as  moist  brown  sugar.  Three  of 
these  batches  made  a  cubic  yard.  It  required  i-J  bbls.  of 
cement  per  cubic  yard  of  concrete.  Cost  per  cubic  yard  con¬ 
crete,  $6.56.” 

This  concrete  was  very  dry.  The  writer  tried  these  propor¬ 
tions  at  the  Susquehanna,  except  the  pebbles,  but  found  the 
concrete  too  dry  to  handle  in  large  quantities  and  rapidly  in 
a  satisfactory  manner,  failed  in  getting  the  stones  uniformly 
distributed  when  rammed  in  place,  and  after  waiting  for  sev¬ 
eral  days  after  depositing  the  concrete  in  the  crib,  found  that 
no  change  whatever  had  taken  place  ;  the  sand  and  cement 
were  still  dry  and  separate,  no  set  whatever  had  taken  place, 
and  the  condition  of  the  mass  was  the  same  as  if  broken  stone 
was  simply  mixed  with  so  much  sand.  After  this  more  water 
was  used,  which  seemed  to  be  very  much  more  satisfactory, 
both  as  to  setting  and  ease  of  handling  and  compacting  into  a 
homogeneous  mass. 

32.  The  proportions  of  cement  to  sand,  and  the  proportions 


FOUNDATIONS. 


19 


of  mortar  to  broken  stone  or  stone  and  gravel  mixed,  seem  to 
vary  in  the  practice  of  engineers  between  wide  limits,  and  all 
apparently  produce  satisfactory  results;  economy  doubtless  in 
most  cases  being  the  most  potent  factor,  but  necessarily  con¬ 
trolled  by  the  size  of  the  stone  used  and  the  manner  of  breaking 
it,  whether  the  stone  is  screened  or  not,  and  the  importance  and 
magnitude  of  the  structure.  In  the  first  caisson  sunk  at  Havre 
de  Grace  the  stone  was  screened,  using  only  the  stone  of  con¬ 
siderable  size.  According  to  records  kept,  we  used  2283  barrels 
of  cement  and  made  1979  cu.  yds.  of  concrete :  this  includes  the 
large  one-man  stone  used,  the  whole  estimated  as  concrete,  or 
I  barrel  of  cement  made  only  about  0.9  cu.  yd.  of  concrete ; 
whereas  the  average  of  the  other  four  caissons,  the  stone  not 
being  screened,  the  average  yield  per  barrel  of  cement  was 
1. 1 5  cu.  yds.  concrete.  The  entire  work  consumed  14,288 
bbls.  of  cement,  mainly  Portland,  and  yielded  14,966  cu.  yds.  of 
concrete,  or  practically  1  bbl.  of  cement  to  1  cu.  yd.  of  con¬ 
crete.  The  unscreened  stone  resembles  closely  the  mass  of 
broken  stone  mixed  with  gravel,  and  requires  proportionately 
less  mortar. 

33.  In  handling  large  masses  of  concrete  an  absolute  rule 
as  to  proportions  would  hardly  lead  to  anything  more  than 
approximately  uniform  results,  as  the  same  crusher  will  vary 
materially  in  the  size  of  the  stone  broken  from  day  to  day,  but 
with  the  same  stone  broken  under  the  same  general  conditions 
the  variation  might  not  be  material.  A  simple  method  of 
determining  the  volume  of  voids  in  a  cubic  yard,  such  as  filling 
a  box  containing  one  cubic  yard  of  the  stone,  after  allowing 
the  stone  to  be  soaked  with  water,  then  pouring  in  water  suffi¬ 
cient  to  fill  the  voids  :  this  volume  of  water  gives  the  volume 
of  mortar  required  to  fill  the  interstices  between  the  stone,  to 
which  a  liberal  excess  should  be  added,  as  it  is  better  to  have 
too  much  than  too  little  mortar.  In  some  cases  mortar  alone 
is  used  to  fill  a  crib.  This  is  expensive,  and  to  save  money, 
mortar  is  thrown  down  in  layers,  and  while  in  this  condition 
large  stones  are  simply  thrown  down  upon  it  at  random  and 
then  another  layer  of  mortar,  and  so  on.  This  necessarily  fails 


r>o 


A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


to  produce  a  homogeneous  mass,  and  unless  the  stones  are 
carefully  placed  they  will  rest  on  each  other,  forming  open 
spaces. 


Article  III. 

USE  OF  CONCRETE. 

34.  There  is  such  a  great  variety  of  purposes  to  which 
concrete  can  be  applied,  that  the  principal  ones  alone  will  be 
mentioned.  It  is  used  to  a  large  extent  and  almost  exclusively 
for  those  parts  of  the  substructure  under  ground  and  under 
water,  in  masses  varying  from  2  feet  to  40  feet  and  more  in- 
thickness. 

In  a  subsequent  article  the  uses  of  concrete  will  be  more 
fully  illustrated.  The  ease  with  which  it  is  applied,  the  ease 
with  which  it  can  be  made  to  conform  to  the  irregularities  of 
the  foundation-bed,  filling  in  under  and  around  the  irregular¬ 
ities,  thus  avoiding  unnecessary  blasting,  hammering,  etc., 
furnishes  the  simplest  and  most  satisfactory  means  of  spread¬ 
ing  the  base  of  the  foundation,  so  as  to  reduce  the  unit 
pressure  on  the  foundation-bed,  and  furnishing  a  uniform 
surface  upon  which  to  build  walls  of  houses,  piers,  abutments, 
and  other  structures;  also  forming  water-tight  floors  and  walls 
for  cellars,  lining  reservoirs,  cisterns;  the  entire  walls  of  houses 
can  be  built  of  it,  and  even  entire  piers,  or  filling  in  piers  faced 
with  masonry,  iron,  or  timber.  In  all  of  these  cases  it  is  in  gen¬ 
eral  more  economical  than  rubble  and  brickwork,  and  certainly 
far  superior  to  brickwork  under  ground  or  under  water. 
Under  walls  of  houses  it  is  commonly  not  used  in  layers 
of  over  1  to  2  feet  in  thickness,  mainly  to  secure  a  base 
wider  than  the  body  of  the  wall  in  order  to  distribute  the 
pressure  over  a  greater  area.  Mr.  Rankine,  in  his  Civil  Engi¬ 
neering,  states  that  the  limit  of  this  widening  depends  upon 
the  depth  of  the  concrete,  viz.  :  Take  a  wall  of  a  house  2 
feet  broad  at  the  base  and  20  feet  long,  this  would  give  40 
square  feet  of  bearing  surface  if  built  directly  on  the  foun¬ 
dation-bed,  but  by  putting  2  feet  of  concrete  and  then  building 


USE  OF  CONCRETE. 


21 


the  wall  you  can  extend  this  concrete  2  feet  on  each  side  of 
the  wall,  forming  a  base  6  feet  broad  and  giving  a  bearing  sur¬ 
face  of  120  square  feet;  if  3  feet  thick  a  bearing  surface  of 
160  sq.  ft. ;  and  so  on.  Upon  this  bed  of  concrete  good  rubble- 
work  is  commonly  built  to  or  a  little  above  the  surface  of  the 
ground,  mainly  as  a  matter  of  economy.  Limestone  is  excellent 
for  this,  and  better  than  sandstone,  although  the  latter  can  be 
and  has  been  used,  either  of  which  is  more  economical  than 
granite.  The  writer  thinks  it  unadvisable  to  use  either  sand¬ 
stone  or  brick  under  the  surface  of  the  ground  unless  cement 
mortar  is  used  ;  in  fact  health,  comfort,  freedom  from  damp¬ 
ness,  demand  cement  to  be  used  below  ground  in  all  cases  ; 
economy  alone  says  lime.  Is  it  not  better  to  be  sure  of  the  best 
foundation  and  economize  in  some  other  part  of  the  structure  ? 
Damp  houses,  cracked  walls,  doors  and  windows  out  of  plumb, 
attest  the  truth  of  the  above ;  and  what  is  more,  how  many 
walls  actually  fall  before  completion  of  the  structure  and  after¬ 
wards,  costing  a  thousand  times  more  than  was  necessary  to 
have  put  the  foundation  in  properly.  Sometimes  timber  is 
laid  on  the  foundation-bed  in  two  layers  crossing  each  other: 
this  is  only  admissible  when  a  permanently  wet  stratum  is 
reached. 

35.  Concrete  is  used  in  large  quantities  under  all  important 
structures,  and  especially  under  bridge  piers,  abutments,  re- 
taining-walls,  etc.,  in  masses  varying  in  depth  from  2  to  40 
feet,  particularly  in  very  deep  foundations,  where  the  pneu¬ 
matic  caisson  is  used.  This  will  be  particularly  alluded  to  when 
we  come  to  discuss  the  subject  of  Pneumatic  Caissons.  It  is 
also  used  to  make  enormous  blocks  of  stone  where,  exposed 
to  the  action  of  immense  moving  forces,  such  as  is  in  exposed 
conditions  on  the  sea-coast,  in  constructing  breakwaters,  it 
would  be  very  difficult  if  not  impossible  to  transport  blocks  of 
the  size  desired.  The  concrete  can  be  manufactured  at  points 
convenient  to  the  site.  Structures  alluded  to  in  the  last  para¬ 
graph  will  be  discussed  more  in  detail  in  another  article. 

36.  On  the  foundation-bed  when  concrete  is  omitted,  or  on 
the  surface  of  the  concrete  when  used,  what  may  be  called  the 


22 


A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


lower  part  of  the  body  of  the  structure  is  constructed.  This 
may  be  of  brick  or  rubble,  and  in  very  large  and  important 
structures  may  be  of  first-class  masonry,  hereafter  to  be  de¬ 
scribed  in  more  detail.  Brick  is  sometimes  used,  and  is  com¬ 
menced  with  one  or  more  footing-courses,  that  is,  courses  pro¬ 
jecting  from  a  quarter  to  almost  one  half  the  length  of  a  brick 
— from  2  to  4  inches.  This  is  not  necessary  when  the  wall 
springs  from  rock  or  a  bed  of  concrete,  as  no  spread  of  base  of 
wall  is  necessary  in  this  case,  but  is  generally  done.  Outside 
bricks  for  projecting  courses  should  be  all  headers :  this  is 
always  done  when  the  walls  spring  from  clay  or  sand;  then 
above  the  footing-courses  the  body  of  the  wall  is  carried  up 
with  the  prescribed  thickness. 

37.  When  this  part  of  the  work  is  of  rubble  the  same  rule 
is  followed,  except  that  the  rubble  wall  is  carried  up  to  the 
surface  of  the  ground  with  a  little  greater  breadth  than  the 
body  of  the  wall  alone,  so  as  to  leave  a  small  offset.  When 
this  part  of  the  wall  is  under  very  large  and  important  struc¬ 
tures,  such  as  bridge  piers,  and  is  made  of  first-class  masonry, 
there  are  generally  several  footing-courses,  forming  a  series 
of  steps  so  arranged  as  to  leave  a  small  offset  just  under  the 
surface  of  the  ground  or  water,  where  the  neat  work  com¬ 
mences.  The  different  kinds  of  masonry  will  be  fully  de¬ 
scribed,  the  proper  kinds  of  bond  and  material  used,  and  all 
technical  terms  used  will  be  explained  in  another  article. 

38.  The  crushing  strength  of  concrete  has  never  been  fully 
determined,  and  in  fact  but  few  experiments  have  been  made. 
Theoretically  it  should  continue  to  harden  indefinitely,  and  all 
that  could  be  done  would  be  to  subject  cubes  or  blocks  to 
compression  (noting  carefully  the  kind  and  the  proportions  of 
ingredients)  after  the  lapse  of  a  certain  time,  and  after  inter¬ 
vals.  This  would  give  us  the  strength  at  that  age,  and  by  com¬ 
parison  the  rate  of  increase  of  strength  ;  but  enough  is  known 
to  establish  the  fact  that  it  will  in  general  acquire  in  a  short 
time  the  strength  of  ordinary  sandstone.  It  is  claimed  by 
some  authorities  that  the  set  or  hardening  is  delayed  by  press¬ 
ure.  For  this  reason  it  is  often  prescribed  that  each  layer 


BUILDING  STONES. 


23 


shall  be  allowed  to  set  before  adding  another  or  before  com¬ 
mencing  the  masonry.  This  cannot  be  done  in  large  struc¬ 
tures  on  account  of  the  delay  that  would  be  caused.  The 
small  amount  of  weight  added  each  day  could  not  cause  any 
trouble. 


Article  IV. 

BUILDING  STONES. 

39.  The  most  important  properties  of  rock  suitable  for 
building  purposes  are  the  Structural  and  Chemical.  In  regard 
to  their  structural  character,  they  are  divided  into  the  unstrati¬ 
fied  and  the  stratified,  or  those  which  show  no  distinct  layers 
or  beds  and  those  that  do  show  such  layers  or  beds  more  or 
less  distinctly.  These  properties  are  of  great  importance,  as  con¬ 
cerns  both  the  strength,  durability,  and  economy  of  structures. 
The  unstratified  rocks  are  generally  the  hardest  and  the  strong¬ 
est,  and  can  be  obtained  in  immensely  large  blocks,  but  at  the 
same  time  are  expensive  to  quarry  and  dress  into  proper 
shapes ;  they  are  compact,  and  have  a  low  absorptive  power ;  all 
of  which  renders  them  valuable  for  structures  of  great  magni¬ 
tude.  Of  these  the  most  common  are  granite  and  syenite.  The 
stratified  rocks  vary  much  in  strength,  durability,  and  compact¬ 
ness,  and  are  formed  in  distinct  layers,  varying  from  the  lami¬ 
nated  or  slaty  structure  in  thickness,  to  that  of  several  feet. 
The  best  kinds  are  hard  and  strong  and  durable,  easily  quar¬ 
ried,  easily  cut  into  desired  shapes,  and  are  widely  distributed 
over  the  country,  and  consequently  are  our  most  useful  and 
common  building  stones,  are  used  in  piers,  retaining-walls,  and 
walls  of  houses.  Being  found  in  many  colors  and  combinations 
of  colors,  they  produce  a  fine  architectural  and  ornamental 
effect ;  of  the  most  common  and  useful  are  marble,  limestone, 
sandstone,  and  slate.  Each  of  these  kinds  will  be  considered 
in  some  detail. 

40.  As  to  the  chemical  composition  of  stones,  they  are 
divided  into  three  classes,  viz.,  silicious,  calcareous,  and  argil- 


24 


A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


laceous  stones,  as  these  several  substances  predominate  The 
pnncpal  silicons  stones  are  granite,  syenite,  and  sandstone  ■  of 
tie  calcareous  stones,  marble  and  compact  limestone  -  of  the 
argillaceous  stones,  clay  slate.  ’  1  1  le 

41.  Granite  is  unstratified  and  silicious,  and  consists  of 
qua!  tz,  feldspar,  mica,  and  hornblende.  Its  valuable  properties 
ate  greater  the  more  quartz  and  hornblende  it  contains  and  less 
in  proportion  to  the  feldspar  and  mica  contained  hm 
to  its  great  hardness  it  is  seldom  used,  except  Jor  h 
lighthouses,  breakwaters,  and  large  public  buddings  Owing 
to  its  great  cost  it  is  seldom  used  in  bridge  niers  „nl '  Z  S 
and  ^  c,iPS 

41  b  Sandstone  is  stratified  and  silicious,  and  is  composed 
°  grains  of  sand  commonly  cemented  together  with  /com 
pound  of  silica,  alumina,  and  lime.  The  best  qualities  of  sand 
stone  are  those  in  which  the  amount  of  cement  tying  the 
material  is  small  and  composed  mainly  of  silica  and  the  ^  • 

a, e  well-defined  and  angular.  Much 

composed  largely  of  alumina  or  lime,  indicates  a  weak  sand 
stone,  and  especially  if  the  grains  are  rounded.  It  exists  ,„‘ 
various  degrees  of  hardness,  compactness,  strength,  and  dura 

b. hty;  ,s  found  of  almost  all  colors,  and  makes  beautiful  and 
ornamental  fronts  to  houses ;  and  being  widely  distributed  it 

stones  O  ^  °T  ^  “Sefu‘  a"d  conv“fc"‘  of  building 

tones  Owing  to  its  more  or  less  distinct  stratification  its 

porosity,  and  consequently  high  absorptive  power,  it  slimild 

always  be  placed  in  structures  on  its  natural  bed.  si  its  layers 

may  be  perpendicular  to  the  direction  of  the  pressure  •  other 

^  t,^Tfdfr°rm  “USe  disi,lte?rati°"  and  scaling 
’  f  ?s  affording  less  resistance  to  the  pressure  -  and  if 

seaU-Ccoa:tmoVStoPre,Sehnt  “  ^  raP''d,y  Whe"  “P°-d  ™  the 

sea  coast  or  to  sulphurous  vapors. 

.  11  is  generaHy  conceded  that  neither  a  physical  exam 

.nation  nor  a  chemical  analysis,  nor  even  an  actual  specimen 
r  crus  ling  strength  of  a  fresh-quarried  stone  gives  even 
an  approximate  idea  of  its  suitability  for  building  purposes; 


BUILDING  STONES. 


25 


but  these  combined  with  some  other  conditions,  such  as  its 
appearance  on  exposed  faces  of  large  masses,  should  in  general 
furnish  satisfactory  indications  of  its  general  properties.  An 
■exposed  face  of  a  mass  of  sandstone  which  we  have  good  rea¬ 
son  to  believe  has  been  exposed  for  a  very  long  time  should 
present  the  following  appearance  :  The  exposed  surface  should 
present  a  hard,  rather  dark-colored  skin,  of  about  an  inch  or 
two  thick;  the  interior  surface  a  little  softer,  and  generally  of  a 
lighter  color :  this  indicates  a  stone  that  hardens  on  exposure. 
All  angle  lines,  vertical  or  horizontal,  should  be  sharp  and  well 
defined.  A  rough  exterior  surface,  with  cavities  of  greater  or 
less  size  and  depth,  with  rounded  corners  or  angle  lines,  indi¬ 
cates  a  soft  variety  of  stone,  and  one  that  wears  and  disin¬ 
tegrates  on  exposure. 

43.  The  writer  examined  the  sandstones  bordering  the 
Ohio  River  for  many  miles  east  and  west  of  Point  Pleasant, 
W.  Va.,  and  also  many  miles  up  the  Kanawha  River,  in  order 
to  select  a  quarry  for  stone  to  be  used  in  a  bridge  at  that 
place ;  and  in  this  limit,  although  finding  many  kinds  different 
in  their  properties,  and  getting  all  information  possible  from 
residents,  he  concluded  that  it  would  not  be  safe  to  risk  the  use 
of  them  in  the  large  and  exposed  piers,  and  it  was  determined 
to  use  the  Hocking  Valley  sandstone  from  a  quarry  over  one 
hundred  miles  distant  by  rail :  this  was  apparently  the  softest 
variety  examined,  was  of  a  dark-brown  color,  and  spawls  could 
be  easily  broken  in  the  hand ;  exposed  surfaces  in  quarries, 
however,  presented  a  good  appearance.  A  block  of  sandstone 
could  be  worked  when  just  from  the  quarry  with  an  ordinary 
pick.  Our  decision,  however,  was  based  on  the  fact  that  we 
found  dams,  piers,  walls  of  houses,  built  of  this  stone,  some  of 
which  we  were  informed  had  been  built  forty  or  fifty  years 
prior  to  this  time,  and  still  bore  the  tool-marks,  and  were  now 
found  to  be  very  hard ;  consequently  we  used  this  stone  to  a 
very  large  extent. 

44.  We  subsequently  found  a  quarry  a  few  miles  up  the 
Kanawha  River.  This  stone  presented  a  favorable  appearance 
in  the  quarry,  and  numbers  of  bowlders,  some  very  large,  were 


26 


A  practical  treatise  oh  foundations. 

found  on  the  hillside  which  seemed  to  be  harder  than  the 
quarry  stone,  and  showing  no  signs  of  disintegration;  conse¬ 
quently  some  of  the  piers  were  built  of  this  stone.  The  bowlders 
vhen  large  enough  were  freely  used ;  the  color  of  this  stone 
was  something  like  rich  cream.  Another  stone  found  near  this 
quarry  on  the  other  side  of  the  river,  of  rather  a  bluish  color 
vvas  extremely  hard  in  the  quarry,  had  a  high  compression 

buTlt  o'f  tl  \  f  qUaiTied’  bLlt  in  Parts  of  structures 
built  of  this  stone  there  were  plain  indications  of  scaling  and 

disintegration:  this  was  used  to  a  very  limited  extent,  and 

mainly  for  backing  stone  and  in  concrete.  These  facts  are 

mentioned  to  show  how  uncertain  appearances  are,  as  well  as 

ie  actual  specimen  test  for  crushing  strength,  unless  the  stone 

has  been  quarried  for  some  time.  It  is  always  desirable  if 

possible,  to  know  that  a  stone  has  stood  the  test  of  .time’ in 

actual  structures  ;  but  often  we  have  to  do  the  best  we  can 

guided  by  such  tests  and  indications  as  above  referred  to. 

45-  Sandstone  may  be  then  divided  into  two  classes :  those 

W]  -V  a-  10Ug  1  S°ft  at  first>  bai-den  on  exposure;  and  those 
which  disintegrate  and  decay  on  exposure,  though  they  may 

be  hard  at  first.  The  first  alone  should  be  used  for  building 

purposes.  Sandstone  stands  exposure  to  fire  better  than 
granite. 

46.  The  writer  collected  a  number  of  samples  from  the 
different  quarries  examined,  and  from  each  two  or  more  sped- 
mens  were  carefully  dressed  into  exact  cubes  2  inches  on  ed^e 
eac  way  and  subjected  them  to  crushing,  using  smoothly 
dressed  wlnte-pine  cushions  cut  of  exact  size  of  the  cube  ;  these 
cushions,  about  one  eighth  to  one  quarter  of  an  inch  thick,  were 
p  aced  on  top  and  bottom  of  cube  to  be  tested.  All  the  sam- 
p  es  tested  by  him  were  strong  enough  to  bear  any  reasonable 
pressure,  varying  from  3000  to  5000  lbs.  per  square  inch,  and 
in  general  even  the  softer  specimens  of  sandstone  will  stand 
the  pressure  ;  but  decay  is  the  great  danger  to  be  avoided. 

47-  Limestones,  stratified  and  calcareous.  Marble  is  gen¬ 
erally  considered  as  a  pure  carbonate  of  lime,  and  is  strong 
and  durable  and  at  the  same  time  easily  cut  and  dressed  ;  and 


BUILDING  STONES. 


27 


from  its  variety  of  color  in  the  same  stone,  as  well  as  the 
variety  of  solid  colors  in  which  it  is  found,  together  with  the 
high  polish  it  will  take,  it  is  largely  used  for  ornamental  pur¬ 
poses,  and  also  in  many  large  public  buildings  as  well  as  in 
private  houses,  but  owing  to  its  great  cost  it  is  not  used  in 
ordinary  structures,  and  in  addition  it  is  not  so  widely  dis¬ 
tributed  ;  yet  marble  quarries  are  claimed  to  exist  in  almost 
every  State  of  the  Union.  Many  limestones  are  susceptible  of 
a  high  polish,  and  present  a  very  beautiful  surface,  and  are 
called  marble  for  this  reason. 

48.  Compact  limestone  is  what  might  be  called  an  impure 
limestone,  containing  greater  or  less  proportions  of  silica, 
alumina,  and  iron,  or  these  combined  ;  and  the  qualities  of  the 
stone  for  building  purposes  depend  more  or  less  upon  the 
amount  of  these  foreign  ingredients.  But,  generally  speaking, 
any  compact,  hard,  and  fine-grained  limestone  is  one  of  the 
most  useful  and  common  building  materials.  A  loose,  porous 
limestone  should  not  be  used ;  however,  some  of  the  soft  varie¬ 
ties  are  found  to  harden  on  exposure.  These  stones  are  often 
difficult  to  quarry  and  dress,  and  often  cannot  be  obtained  in 
anything  like  regular  shapes,  and  are  therefore  useless  for  any¬ 
thing  but  rubble  work.  Other  varieties  occur  in  well-defined 
layers  of  thicknesses  from  1  inch  to  2  feet,  are  easily  quarried, 
require  but  little  dressing,  and  are  both  economical  and  dura¬ 
ble;  should  always  be  laid  on  their  natural  beds,  and  no  excuse 
can  exist  for  not  doing  so  (in  sandstones  it  is  difficult  to  deter¬ 
mine  in  some  varieties  which  is  the  natural  bed).  Its  absorp¬ 
tive  power  is  small,  and  therefore  it  is  not  liable  to  disintegrate 
by  action  of  frost.  It  will  not  stand  exposure  to  high  tempera¬ 
ture,  and  is  rapidly  disintegrated  in  case  of  fires  in  cities.  The 
pure  varieties  of  limestone,  when  properly  burned,  yield  the 
ordinary  quicklime,  and  those  which  contain  certain  deter¬ 
mined  proportions  of  silica  and  alumina  yield  hydraulic  limes. 
Limestones  effervesce  with  acids — a  distinguishing  charac¬ 
teristic. 

49.  Argillaceous  Stones.  The  only  variety  of  these  stones 
of  any  value  to  the  engineer  is  what  is  known  as  slate.  Its 


28 


A  PRACTICAL  treatise  on  foundations. 

principal  use  is  for  roofing  houses.  This  is  a  stratified  stone  and 
when  ,t  can  be  split  into  very  thin  layers  it  has  what  is  said  to 
be  a  laminated  structure.  It  is  found  of  several  colors,  but  the 
darker  colors  in  general  indicate  great  strength  and  durability, 
it  is  almost  impervious  to  water.  J 

5°.  A  table  of  the  resistance  to  crushing  of  these  several 
kinds  of  stone  has  already  been  given.  The  absorptive  power 
of  these  stones  can  be  arranged  according  to  a  descending 
scale  as  follows :  Sandstone,  compact  limestone,  marble,  and 
granite,  the  two  last  practically  absorbing  no  water  at  all 
The  absorptive  power  can  be  easily  determined  by  weighing  a 
specimen  dry,  and  then,  after  being  immersed  in  water  for  a 
reasonable  time,  the  increase  of  weight  will  determine  the 
amount  of  water  absorbed.  After  removing  from  the  water, 
the  surface  water  adhering  should  be  allowed  to  drip  off.  As 
to  resistance  to  heat,  the  order  may  be  taken  as  follows :  Sand¬ 
stone,  granite,  limestone,  the  last  being  entirely  decomposed 
under  the  influence  of  intense  heat. 


Article  V. 

QUARRYING  AND  STONE-CUTTING. 

51.  It  has  been  formerly  stated  that  it  is  the  duty  of  engi¬ 
neers  to  design  and  build  structures  suitable  to  the  purposed 
view,  and  it  is  easily  in  the  recollection  of  the  present  venera¬ 
tion  when  the  engineer,  so  called,  was  expected  to  know  how 
to  do  almost  everything  in  the  way  of  utilizing  and  controlling 
the  forces  and  materials  of  nature,  in  promoting  the  comfort^ 
happiness,  and  prosperity  of  mankind  ;  and  as  at  the  period 
refeired  to  but  little  was  known,  it  was  possible  for  one  man  to 
know  and  to  put  into  practice  what  was  known, — mainly  by  a 
soit  of  rule-of-thumb  method.  This  perhaps  may  have  originated 
the  prefix  Civil  to  the  general  term  engineer.  But  in  the  past 
few  years  such  development  and  progress  has  been  made  in 
the  sciences  and  arts,  that  it  has  become  necessary  to  divide 
the  subject  into  almost  numberless  branches,  all  more  or  less 


QUARRYING  AND  STONE-CUTTING. 


29 


allied  and  interlinked,  but  each  so  broad  and  deep  that  he  is 
fortunate  who  has  the  time  to  master  any  one  of  its  subdivi¬ 
sions  ;  and  here  we  have  the  civil,  the  mechanical,  the  hydraulic, 
the  city,  and  now  the  electrical  engineer,  to  say  nothing  of  the 
architect  and  the  bridge  engineer.  Bridge  construction  has  now 
become  an  almost  exclusive  science.  Consequently  it  is  difficult 
to  know  how  much  of  each  of  these  any  one  should  know,  in 
order  to  claim  or  deserve  either  of  the  above  titles,  and  equally 
difficult  to  determine  the  border-line  between  any  two  of  them. 

52.  These  considerations  must  be  the  writer’s  excuse  for 
introducing  several  subjects  that  would  seem  to  have  not  the 
least  connection  with  what  he  shall  give  as  a  title  to  this  vol¬ 
ume,  namely,  a  Treatise  on  Foundations,  and  must  at  the  same 
time  explain  the  omission  of  many  things  that  should  be  in¬ 
cluded. 


QUARRYING. 

53.  Quarrying  is  purely  an  art,  and  little  can  be  learned  of 
it  except  by  experience.  The  illiterate  quarryman  will  take 
out  more  stone,  and  in  better  shape,  in  twenty-four  hours,  than 
the  ordinary  engineer  will  do  in  a  month  ;  but  still  it  seems 
that  he  should  at  least  have  the  benefit  of  the  few  general 
principles  that  are  known.  All  stones,  even  the  granite,  have, 
to  the  expert,  well-marked  division  lines ;  limestone  and 
sandstone  have  them  generally  well  defined,  and  the  first  prin¬ 
ciple  in  quarrying  should  be  to  detect  these  division  lines,  not 
only  as  a  matter  of  economy,  but  also  to  obtain  the  blocks  of 
the  proper  size  and  shape.  Another  principle  is  either  to  use 
no  powder  or  very  little  explosive  material,  except  in  case  of 
the  very  hardest  kind  of  rocks,  such  as  granite,  and  then  with 
great  care  and  judgment,  as  it  is  hard  to  determine  the  effect 
of  an  explosion  upon  the  portions  of  the  mass  loosened,  it 
may  produce  injurious  effects,  which  may  remain  unseen  and 
seriously  impair  the  ultimate  strength  and  durability  of  the 
material.  However,  blasting  with  powder  or  dynamite  is  usually 
resorted  to,  the  large  volumes  loosened  and  time  saved  com¬ 
pensating  for  the  waste  caused  by  the  explosion,  and  in  addition 


30 


A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


a  judicious  use  of  small  charges  seem  to  produce  better  results 
than  larger  charges.  Limestone  in  layers  can  -enerallv  he 
quamed  by  the  use  of  picks,  crowbars,  hammers,  "and  wed-es 

.’d,K°n"  Can  often  be  readily  quarried  by  the  same  tools 

k  ed  by  the  use  of  the  plug  and  feathers,  which  consist  of  a 
small  stee  wedge  and  two  iron  semi-cylindrical  pieces  ;  but  un 
less  the  stratification  is  well  defined,  blasting  is  resorted  to  • 

bLk°  tC'  1 13  f°Und.advantageous  to  throw  down  very  large 
blocks  oi  the  material,  and  subsequently  subdivide  theS 
either  by  small  blasts,  or  by  the  use  of  the  above  tools.  The 
p  ugs  and  feathers  are  used  by  first  drilling  a  series  of  small 
holes  a  few  inches  deep  in  a  line,  then  placing  two  feathers  in 
each  hole  and  driving  the  plugs  between  them.  No  attempt  is 
ma  e  to  drive  each  plug  or  wedge  any  great  depth  at  any  one 
time,  but  a  blow  of  a  hammer  is  given  in  succession  on  each 
p  ug  m  the  line,  and  the  stone  will  soon  split  entirely  through 
the  block  along  the  line  of  the  holes. 

54-  When  blasting  is  necessary,  holes  have  to  be  drilled  of 
greater  or  less  depth,  and  varying  in  diameter  from  i*  to  2i 
inches  These  holes  are  then  partly  filled  with  a"  large¬ 
grained  powder  or  dynamite,  and  exploded  either  by  ordi- 
naiy  fuse  or  electricity;  several,  at  distances  apart  depending 
upon  circumstances,  are  fired  simultaneously,  and  at  definitf 
imes,  such  as  at  noon  and  at  the  end  of  the  day,  when 
the  men  can  be  away  at  meals,  in  order  to  have  plenty  of 
work  ready  when  they  return.  There  seems  to  be  no  fixed 
rule  as  to  amount  of  explosive  material  used,  as  conditions 
vary  greatly,  even  in  the  same  quarry,  and  nothing  but 
expenence  and  good  judgment  can  be  depended  upon;  an 
ok, nary  rule  is  to  fill  the  hole  about  one-third  full  of  powder 
The  hole  should  then  be  filled  by  first  placing  a  few  inches  of 
dry  clay  on  top  of  the  powder.  This  clay  should  be  free  from 
sand  or  grit,  and  should  be  gently  tamped  or  compacted  with 
a  wooden  rammer,  to  avoid  premature  explosion.  The  hole 
can  then  be  filled  with  sand  or  other  rubbish.  Results  seem 
to  show  that  from  *  to  2*  pounds  of  powder  are  required 
to  loosen  thoroughly  a  cubic  yard  of  rock  in  place  4s 


QUARRYING  AND  STONE-CUTTING. 


31 


generally  stated,  the  mass  of  rock  loosened  bears  some  propor¬ 
tion  to  the  line  of  least  resistance  cubed,  and  estimated  at 
about  twice  that  result,  it  being  understood  that  that  line  is 
the  shortest  distance  to  the  exposed  face  of  the  rock  from  the 
charge  ;  but  this  is  often  far  from  the  fact,  as  this  least  re¬ 
sistance  depends  upon  the  nature  and  character  of  the  material, 
the  position  and  direction  of  the  hole  and  the  manner  of 
tamping  or  filling  the  hole. 

55.  The  holes  can  be  drilled  or  bored  by  hand  or  machinery. 
There  are  three  methods  by  hand.  In  the  first,  a  long  iron  rod, 
with  a  steel  chisel-shaped  cutting  edge,  is  lifted  by  one  or  two 
men  to  a  certain  height  and  then  allowed  to  drop  in  the  hole, 
giving  a  slight  turn  after  each  blow.  In  the  second,  an  iron 
rod  of  varying  lengths,  according  to  the  depth  of  the  hole 
required,  is  held  by  one  man,  and  two  men  strike  on  the  top  of 
the  drill  alternately,  the  man  holding  the  drill  turning  it  con¬ 
tinuously  as  the  blows  are  struck.  The  first  of  these  is  consid¬ 
ered  more  efficient.  In  the  third,  known  in  practice  as  “  ball 
drilling,”  one  man  has  a  long  iron  rod,  with  a  specially  made 
point,  this  rod  he  simply  lifts  and  throws  into  the  hole,  as  it 
were.  The  accuracy  with  which  they  handle  the  drill  and  the 
rapidity  of  the  work  are  certainly  astonishing,  and  perhaps  the 
reason  that  it  is  so  seldom  resorted  to  is  that  it  requires  the 
skill  of  a  drum  major  to  keep  the  hole  straight  and  hit  in  it 
every  time.  A  day’s  work  in  drilling  will  vary  from  5  to  15 
feet  per  man. 

56.  Machine-drilling  is  on  the  same  general  principles,  ex¬ 
cept  the  power  is  applied  by  steam.  The  drills  are  moved 
forward  by  blows  or  turning,  or  both,  and  of  course  on  exten¬ 
sive  works  progress  is  more  rapid  and  economical  than  by  hand¬ 
drilling.  The  diamond  drill  is  in  very  common  use,  is  expensive 
in  its  first  cost,  and  rarely  used  when  limited  quantities  of  ma¬ 
terial  are  to  be  quarried.  The  tube  or  drill  in  this  case  is  a  pipe 
or  hollow  tube,  having  a  head  at  the  bottom,  in  which  is  placed 
number  of  small  black  diamonds,  projecting  slightly  from  the 
surface.  This  is  caused  to  revolve  rapidly  and  cuts  a  cylindri¬ 
cal  hole.  The  material,  in  the  form  of  dust  or  small  particles 


32 


A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


of  the  stone,  is  brought  to  the  surface  by  forcing  water  down 
the  tube  through  holes  in  the  head  and  returning  through  other 
channels  on  the  outside  of  the  drill.  In  hand-drilling  the 
debris  or  dust  is  removed  in  a  very  crude  way- — by  first  remov¬ 
ing  the  drill  from  the  hole  and  inserting  a  long  branch  of  some 
kind  of  wood,  split  and  broomed  at  the  end,  or  by  the  use  of 
small  wooden  or  iron  spoons,  connected  to  the  end  of  a  pole. 
During  the  process  of  drilling,  water  has  to  be  continually 
poured  into  the  hole.  It  aids  the  drilling  by  softening  to  some 
extent  the  material,  and  keeps  the  end  of  the  drill  cool. 

57-  Dynamite  has  many  times  the  explosive  power  of  pow¬ 
der,  varying  according  to  the  percentage  of  nitro-glycerine  it 
contains,  and  is  generally  used  in  place  of  powder  when  a  vio¬ 
lent  and  sudden  explosion  is  required,  as  blasting  in  railroad 
cuts  or  in  removing  large  masses,  regardless  of  the  shape  or 
size  in  which  they  are  thrown  down  ;  but  in  quarrying  for 
dimension  stone  great  care  should  be  used  to  avoid  too  much 
shattering  of  the  stone  and  breaking  into  small  pieces.  Dyna¬ 
mite  generally  is  sold  in  candles,  so  called,  of  almost  any 
diameter  and  length,  and  containing  different  quantities  by 
weight,  wrapped  in  brown  paper,  which  makes  them  convenient 
to  handle,  and  apparently  no  more  dangerous  than  powder,  as 
certainly  the  men  handle  it  as  carelessly  as  they  do  the  ordi¬ 
nary  blasting-powder. 

58.  Quarries  should  always,  when  practicable,  be  opened  on 
hillsides,  so  as  to  obtain  a  large  vertical  working  face,  and  the 
top  soil  stripped  off  until  a  solid  ledge  is  reached  over  a  con¬ 
siderable  area.  This  stripping  is  generally  expensive  in  first 
cost.  This  stripping  can  be  done  economically  and  rapidly 
with  a  water  jet  where  water  in  sufficient  quantities  is  con¬ 
venient,  but  the  necessary  machinery  is  expensive. 

59-  The  most  economical  condition  for  quarrying  is  when 
all  of  the  stone,  both  large  and  small,  can  be  utilized,  as  otherwise 
the  waste  will  be  very  great.  All  things  considered,  the  cost 
of  the  construction  will  largely  depend  on  this,  as  in  order  to  get 
dimension  or  face  stone  for  piers  there  will  necessarily  be  a 
large  quantity  of  large  stone  unfit  for  face  stone,  and  at  the 


QUARRYING  AND  STONE-CUTTING. 


33 


same  time  a  large  quantity  of  small  stone,  such  as  one-man 
stone,  and  spawls  suitable  for  breaking  into  stones  for  concrete. 
Consequently,  if  a  series  of  bridge  piers  can  be  so  planned  as 
to  combine  in  the  same  pier  all  of  these  sizes  and  shapes,  the 
cost  of  construction  would  evidently  be  lessened.  In  many 
cases  this  can  be  done  consistently  with  the  recognized  and 
good  practice,  the  broken  stone  and  one-man  stone  used 
under  ground  and  under  water,  and  the  large,  rough  stone  used 
for  backing  in  the  piers :  or  in  some  of  the  piers  the  backing 
could  be  large  stone  and  in  others  concrete ;  or  even  a  combi¬ 
nation  of  these  in  the  same  pier,  alternating  the  courses,  one 
course  backed  with  large  stone  and  another  backed  with  con¬ 
crete,  the  latter  producing  seemingly  a  stronger  pier  than  that 
built  by  either  of  the  other  methods.  Absolute  uniformity  is 
the  common  practice,  and  dependent,  as  has  been  stated,  prac¬ 
tically,  on  the  whim  of  the  chief  engineer.  Surely  common- 
sense  would  justify  the  combination  pier,  with  knowledge  before 
us  that  either  independently  has  been  used  repeatedly  and  with 
satisfactory  results.  (See  Plate  II,  Figs.  I  and  2.)  Some  engi¬ 
neers  will  not  allow  the  backing  stone  to  be  of  a  different  kind 
from  the  face  stone,  when  either  are  recognized  as  good  enough 
for  the  entire  structure.  One  reason  assigned  is  that  different 
kinds  of  stone  have  different  degrees  of  expansion  and  con¬ 
traction  under  changes  in  temperature.  Probably  the  greatest 
differences  in  hardness  and  strength  exist  in  granite  and  sand¬ 
stone.  According  to  Rankine,  granite  expands  .0009  of  its 
length  in  a  change  of  temperature  of  180°  Fahr.,  and  sand¬ 
stone  varies  in  the  same  range  from  .0009  to  .0012  of  its  length. 
Or  take  90°  as  probably  the  greatest  possible  range  of  tempera¬ 
ture  likely  to  occur,  and  we  have  for  extreme  differences 
.00045  and  .0006.  But  this  range  of  temperature  in  a  mass  of 
masonry  is  improbable,  and  the  fact  is  that  the  expansion  is 
microscopic. 

60.  Many  engineers  put  upon  themselves  onerous  and  often 
useless,  if  not  harmful,  duties,  such  as  specifying  for  each  pier 
of  a  bridge  the  exact  size  of  each  stone  in  a  pier  and  in  each 
course.  This  necessarily  leads  to  delay,  confusion,  and  expense. 


34 


A  PR  ACT/ CAL  TREATISE  ON  FOUNDATIONS. 


A  good  quarry  foreman  always  makes  a  diagram  of  each  course 
in  a  pier,  and  can  easily  select  from  the  supply  in  the  yard 
such  stones  as  will  fulfil  the  conditions  of  good  masonry,  which 
are  marked  and  forwarded,  together  with  the  diagram,  to  the 
site  of  the  work,  and  only  occasionally  requiring  any  cutting, 
except  for  a  closure,  unless  in  case  of  rejection  of  the  stone 
when  delivered.  These  lengths  and  sizes  may  vary  slightly 
from  any  arrangement  that  would  be  made  by  the  engineer, 
but  in  ordinary  massive  masonry  would  present  as  good  an 
appearance  and  have  equal  bond.  The  proper  rule  is  to  fix 
definitely  your  limits  upon  the  sizes,  extent  of  bond,  propor¬ 
tions  of  headers  and  stretchers,  and  allow  reasonable  variations 
between  them.  Harmony  will  prevail,  good  work  be  secured, 
and  money  be  saved.  Onerous  requirements,  especially  when 
evidently  useless,  produce  often  the  exactly  opposite  result. 

6l.  Almost  all  large  and  important  works  are  done  by  con¬ 
tract,  for  the  obvious  reason  that,  all  things  considered,  they  can 
be  done  more  cheaply  and  more  expeditiously  in  this  way;  and 
although  the  writer  has  met  with  rascals  in  almost  all  depart¬ 
ments  of  the  contracting  business,  he  is  glad  to  say  that  he  is 
not  one  of  those  who  think  that  all  or  even  a  large  majority  of 
them  can  be  considered  as  belonging  to  that  class.  On  the 
contrary,  he  believes  that  they  are  otherwise;  and  he  would 
rather  have  a  reliable  contracting  firm  to  do  work  without  close 
inspection,  if  the  firm  has  confidence  in  his  justice  and  good 
judgment,  with  reasonable  requirements,  than  to  conduct  the 
work  in  accordance  with  the  most  onerous  requirements  and 
most  rigid  and  ruthless  inspection  without  such  confidence. 


Art.  VI. 

STEREOTOMY. 

62.  Stereotomy,  as  the  art  of  stone-cutting  is  called,  is  an 
important  and  interesting  subject,  as  well  as  a  difficult  one  in 
practice;  but  it  rather  belongs  to  the  domain  of  the  architect 
than  that  of  the  engineer,  and  except  in  the  ornamental  parts 


STEREO  TOM  V. 


35 


of  structures,  the  forms  used  are  simple  and  we  may  say,  few 
in  number.  In  ordinary  and  massive  structures  the  surfaces 
are  plane  or  cylindrical,  circular  or  elliptic,  and  all  the  stones 
in  the  same  structure  are  generally  of  the  same  shape  where 
plane  surfaces  are  departed  from.  The  true  “skew  arch” 
is  an  exception,  every  stone  having  a  different  shape  and 
size,  and  of  several  kinds  of  curved  surfaces;  but  as  com¬ 
monly  built  the  surfaces  are  either  plane  or  cylindrical.  The 
more  ornamental  parts  of  a  structure  require  a  profound 
knowledge  of  forms  and  combinations  of  forms,  geometrical 
shapes  and  lines  and  the  manner  of  constructing  them  on 
paper,  templets  and  models ;  the  skilled  stone-cutter  does 
the  balance,  and  for  the  simple  forms  the  more  intelligent  of 
these  can  do  it  all  with  a  little  aid  in  calculating  the  radii 
necessary.  In  ordinary  structures  all  the  stones  have  plane 
surfaces  and  the  angles  are  right  angles.  In  piers  in  rivers 
the  ends  are  sometimes  cylindrical,  circular,  or  elliptical  or 
wedge-shaped,  and  always  when  exposed  to  heavy  flows  of  drift 
or  ice.  In  arches  the  sides  are  plain.  The  bottom  is  a  part  of 
the  surface  of  a  cylinder;  the  top  is  generally  left  rough. 
The  whole  stone  is  the  frustum  of  a  wedge,  the  sides  being 
slightly  inclined,  so  as  to  conform  to  the  direction  of  the  radii 
of  the  arch.  Ordinary  templets  or  models  used  in  cutting  the 
surfaces  of  the  stone  are  made  of  wood. 

63.  The  tools  used  by  the  stone-cutter  are  hammers  of 
various  sizes  and  weights — both  ends  blunt,  or  one  end  chisel¬ 
shaped  or  pointed,  or  both  ends  chisel-shaped  or  pointed, 
and  some  patent  hammers,  and  in  addition  tools  called  the 
point  and  chisel.  Stone-cutters  generally  provide  their  own 
tools. 

64.  The  work  in  general  is  performed  by  first  cutting  chisel- 
drafts  around  the  edges  of  the  stone  about  inches  wide — 
these  all  in  the  same  plane ;  and  by  the  aid  of  a  straight-edge, 
a  piece  of  timber  about  6  feet  long,  3  inches  wide,  and  1 
inch  thick,  the  enclosed  rough  stone  is  dressed  down  to  the 
same  plane.  For  a  curved  surface  two  chisel-drafts  are  cut, 
one  at  each  end  of  the  stone,  to  conform  to  the  templet,  and 


36  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


the  intermediate  rough  stone  cut  out  in  the  same  manner. 
Intermediate  chisel-drafts  are  cut  if  the  stones  are  very  large. 
Stone-cutters  are  generally  charged  with  the  loss,  if  by  careless¬ 
ness  they  ruin  stone  in  cutting. 

65=  It  is  generally  prescribed  that  the  beds  and  joints  shall 
be  plane  surfaces  at  right  angles  with  each  other.  This  is 
seldom  fully  realized  in  practice,  and  a  very  slight  concavity  is 
in  some  respects  favorable.  The  straight-edge  should  be 
applied  longitudinally,  transversely,  and  diagonally  to  see  that 
the  stone  is  out  of  wind  or  not  warped,  and  the  surface  of 
the  stone  should  closely  conform  to  the  straight-edge.  It  is 
also  advisable  to  dress  the  side  joints  a  little  slack;  that  is,  if 
stones  2  feet  broad  on  bed  are  placed  touching  on  the  face, 
they  should  be  open  from  \  to  £  inch  at  the  back :  this  favors 
filling  the  joints  easily.  Stones  are  not  required  to  be  abso¬ 
lutely  of  the  same  width  from  face  to  back,  and  for  the  entire 
depth  of  the  stone ;  this  would  be  called  close  dimension 
stone,  but  it  is  generally  specified  that  they  shall  be  of  the 
same  width  for  1  ft.  to  ft.  from  the  face.  The  bottom  bed 
of  a  stone  should  be  cut  strictly  to  the  same  plane  over  its 
entire  surface ;  the  top  bed  may  have  slight  inequalities  on  its 
surface,  as  they  will  be  necessarily  filled  with  mortar,  and 
it  is  generally  allowed  that  the  large  backing  stones  may 
have  from  to  1  inch  less  thickness  than  the  face  stones,  but 
should  in  general  have  almost  as  good  beds. 

66.  Sometimes  a  chisel-draft  is  required  to  be  cut  around 
the  edges  of  the  stones,  to  enable  the  mason  to  set  the  stones 
exactly  over  each  other.  A  good  clean-cut,  straight  pitch-line 
will  answer  fully  for  this  purpose  and  cost  less,  but  it  is  advis¬ 
able  generally  to  cut  this  draft  at  the  angles  or  corners  of  piers  ; 
but  this  is  not  always  done.  The  writer  always  determined 
the  exact  centre  and  laid  off  the  masonry  to  calculated  dimen¬ 
sions  every  fourth  or  fifth  course,  so  as  to  avoid  any  possibility 
of  the  pier  getting  out  of  plumb. 

67.  Stone-cutters  are  very  apt  to  cut  the  stone  so  that  it 
will  not  be  as  thick  on  the  back  as  it  is  on  the  face.  This 
should  not  be  allowed,  as  it  makes  the  mortar  joint  too  thick 


MASONRY. 


37 


under  the  stone.  This  should  be  carefully  measured  with  a 
rule,  or  better  with  an  instrument  made  as  follows  (Fig.  3) :  A 
batten  3  or  4  feet  long  with  a  projecting  piece  at  the  bottom, 
and  a  sliding  piece  attached ;  the  projecting  piece  is  placed 
under  and  against  the  stone;  the  sliding  piece  is  then  lowered 
to  touch  the  stone  on  top  and  fastened  ;  this  scale  is  then 
applied  to  several  points  front  and  back,  which  will  readily 
show  any  variation  in  the  thickness.  The  face  stones  in  each 
course  should  have  absolutely  the  same  thickness  or  rise  of  the 
course.  In  most  massive  structures  the  face  of  the  stone  is 
generally  left  rough  or  rock  face,  and  generally  the  extent  of 
the  projections  is  immaterial,  but  it  is  usual  to  limit  it  to  4 


Fig.  3.— Gauge  for  Sizing  Stone. 

or  5  inches;  but  the  ends  of  piers  below  high-water  and  for 
some  distance  above,  where  there  is  much  drift  or  ice,  should 
be  dressed  to  a  reasonably  smooth  surface,  or  even  bush- 
hammered, — that  is,  dressed  as  smooth  as  possible, — and  this 
should  extend  below  the  water.  The  stones  are  all  cut  to  the 
proper  batter  in  the  yards,  except  for  the  stones  of  the  foot¬ 
ing-courses.  The  stones  for  each  pier  are  generally  cut  in 
advance  of  the  building,  and  piled  up  at  some  convenient  place, 
arranged  according  to  courses  as  far  as  practicable,  increasing 
in  thickness  of  courses  from  the  bottom  to  the  top, — the  inverse 
order  from  that  in  which  they  are  to  be  used  in  the  structure, 
— in  order  to  avoid  too  much  labor  in  handling. 

Article  VII. 

MASONRY. 

68.  It  will  be  best  to  adhere  strictly  to  the  common  classifica¬ 
tions,  as  generally  understood  in  this  and  other  countries.  We 
shall,  however,  reverse  the  general  order  and  commence  with 
the  inferior  kind,  as  follows :  Dry  stone  walls,  ordinary  rough 


38  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 

rubble  walls,  rubble  walls  in  course,  block-in-course  masonry, 
ashlar  masonry.  There  are  also  some  combinations  of  these,  as 
walls  made  with  ashlar  or  block-in-course  or  brick,  and  backed 
up  with  rubble.  The  most  usual  and  widely  distributed  stones 
for  building  purposes  are  granite,  marble,  limestone,  sand¬ 
stone,  and  brick. 

69.  Granite,  owing  to  its  extreme  hardness,  is  seldom  used 
except  in  the  most  important  structures,  such  as  lighthouses, 
large  piers,  and  public  buildings,  when  cost  of  construction  is 
not  considered.  Marble,  though  not  so  hard,  and  can  easily 
be  worked  into  ornamental  shapes,  is  likewise  only  used  in 
buildings  where  the  cost  is  ignored.  Therefore  for  ordinary 
purposes  we  are  compelled  to  rely  upon  the  following  stones. 

70.  Limestone  is  one  of  the  most  useful,  most  generally 
used  and  widely  distributed  of  the  building  materials, 
and  can  generally  be  relied  upon.  It  is  found  in  various 
conditions  of  stratification,  from  the  gnarled  and  twisted  to 
that  of  the  most  perfect  layers,  in  various  thicknesses  from  a 
few  inches  to  two  or  more  feet.  In  this  condition  it  is  easily 
quarried,  comes  out  with  good  beds,  requiring  little  or  no 
labor  in  dressing  and  cutting,  and  can  be  gotten  of  almost  any 
length  and  breadth.  Its  strength  and  durability  depends  upon 
its  compactness.  It  will  not  stand  a  high  heat,  under  which, 
it  disintegrates,  and  also  when  exposed  to  an  acid  atmosphere. 

71.  Sandstone  is  also  widely  distributed,  strongand  durable, 
and  can  easily  be  cut,  sawed,  and  dressed  ;  occurs  in  thick  strata, 
and  can  easily  be  quarried  in  blocks  of  almost  any  dimensions, 
all  of  which  conditions  render  it  a  useful  and  valuable  build¬ 
ing  material  for  almost  any  kind  of  structures,  but  withal  one 
of  the  most  uncertain  and  treacherous  of  stones,  as  it  exists  in 
all  conditions  of  compactness  and  hardness;  but  unfortunately 
the  hardest  varieties  when  first  quarried  may  be  the  least 
durable,  and  some  of  the  softest  varieties,  which  can  be  dressed 
with  a  pick  when  first  quarried,  prove  ultimately  the  most  dura¬ 
ble.  Those  varieties  which  present  sharp  grains  with  a  small 
amount  of  cementing  material  are  generally  the  best.  The 
safest  plan,  however,  is  to  examine  structures,  chimneys,  steps, 


MASONRY. 


39 


etc.,  built  of  this  material  and  known  to  have  stood  for  a  long 
period  of  time.  These  can  generally  be  found,  but  in  the  ab¬ 
sence  of  this  guide  we  have  to  do  the  best  we  can.  Sandstone 
is  porous,  and  special  care  should  be  taken  to  build  it  on  its 
natural  bed,  but  in  many  varieties  of  sandstone  it  is  hard  to 
determine  the  direction  of  the  stratification.  Mineralogy  will 
give  the  color,  general  appearance,  and  locality  where  found, 
and  other  general  properties.  Chemistry  will  enable  the 
reader  to  determine  the  exact  composition,  and  engineers 
should  be  reasonably  familiar  with  these  subjects. 

72.  Dry  stone  walls,  although  not  capable  of  bearing  any 
great  weight,  unless  constructed  of  regular-shaped  stone,  with 
good  beds,  are  useful  for  retaining-walls  of  small  height,  and 
can  be  built  of  almost  any  shape  and  size  of  stone,  and  even 
of  round  river  jacks  or  bowlders,  and  answer  well  in  those  cases 
where  no  danger  or  risk  could  occur  if  they  did  fall  down,  and 
where  great  economy  is  desired. 

73.  Rough  rubble  masonry  is  built  of  any  shaped  stones,  just 
as  they  may  come  from  the  quarry,  without  hammering  or  any 
kind  of  dressing;  but  generally  one  or  two  man  stones  down 
to  the  smaller  spawls  are  laid  without  regard  to  continuous 
horizontal  joints  or  beds,  but  with  special  care  to  breaking 
joints  vertically,  by  overlapping  the  stones,  producing  what  is 
called  “  bond,”  and  well  bedded  in  mortar,  generally  of  common 
lime  and  sand  ;  vertical  joints  are  also  filled  with  mortar,  and 
any  openings  between  the  larger  stones  on  the  beds  or  joints 
should  be  filled  with  smaller  stones  bedded  in  mortar.  Thus 
built,  it  will  make  a  wall  of  considerable  strength,  especially  if 
built  with  cement  mortar,  and  in  the  latter  case  will  make  a 
good  arch  ring  for  small  arches,  its  strength  somewhat  exceeding 
the  strength  of  the  mortar  used  when  hardened  ;  and  when 
faced  with  a  good  coating  of  stucco  or  cement  mortar,  can  be 
made  to  present  a  neat  face.  This  kind  of  work  is  used  in  the 
lower  part  of  foundations  to  carry  even  very  heavy  loads,  and 
is  suitable  for  ordinary  retaining-walls,  and  for  many  purposes 
where  economy  is  a  matter  of  importance. 

74.  Rubble  walls  in  courses.  In  this  class  of  work  there 


40 


A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


are  no  regular  courses  of  uniform  thickness,  the  joints  between 
the  stones,  both  in  vertical  and  horizontal  planes,  being  broken. 
The  side  joints  need  not  be  vertical,  and  the  stones  may  be 
only  hammer-dressed  on  joints  and  bed  ;  but  with  good  mortar 
and  reasonable  care  in  building  so  as  to  have  a  good  bond,  this 
class  of  work  can  be  made  strong  and  durable,  and,  where 
looks  are  not  considered,  would  answer  almost  any  ordinary 
requirement,  and  may  be  made  to  harmonize  pleasantly  with 
rustic  surroundings,  and  possesses  one  important  element — 
economy.  To  a  large  extent  the  sizes  of  the  stones  used  are 
unimportant,  from  very  large  to  very  small.  It  is  the  kind  of 
masonry  almost  exclusively  used  for  backing  retaining-walls. 

75.  A  better  class  of  this  kind  of  work,  in  which  the  beds 
and  joints  are  dressed,  makes  a  strong  and  durable  structure. 
The  irregularity  in  the  size  and  shape  of  the  stones,  provided 
the  joints  between  the  stones  are  broken  horizontally  and 
vertically,  the  rough  undressed  face  of  the  stone,  all  com¬ 
bine  to  produce  a  fine  architectural  effect ;  some  of  the 
handsomest  churches  and  other  structures  are  built  of  this 
class  of  masonry,  though  hardly  to  be  recommended  for  heavy 
structures  or  structures  subjected  to  forces  tending  to  drag  or 
knock  the  smaller  stones  out  of  place,  such  as  bridge  piers, 
which  have  to  stand  blows  and  shocks  from  driftwood,  ice,  etc., 
will  form  nevertheless  a  substantial  and  economical  structure 
for  ordinary  purposes. 

76.  To  build  this  class  of  work  great  care  must  be  taken  to 
secure  good  bond,  both  longitudinally  and  transversely,  and 
due  care  should  be  given  to  proper  adjustment  and  distribu¬ 
tion,  over  the  entire  surface,  of  the  larger  and  smaller  stones. 

77.  Block-in-course  work.  This  class  of  work  varies  from 
the  above  in  having  regular  courses  of  uniform  thickness, 
varying  from  six  to  ten  inches.  The  stones  are  cut  into  regular 
blocks  of  prescribed  length  and  breadth,  the  length  about  three 
times  the  thickness  and  the  breadth  from  one  to  two  times  the 
thickness,  beds  and  joints  cut  true  and  at  right  angles  to  each 
other.  About  one  fourth  of  the  faces  should  show  headers, — that 
is,  stones  whose  ends  show  on  the  face  of  the  wall  and  extend 


MASONRY. 


41 


at  least  three  times  the  depth  of  the  course  into  the  wall,  the 
breadth  of  the  headers  being  at  least  equal  to  the  thickness  of 
the  course,-— and  they  should  rest  on  the  stretchers  below  as 
nearly  over  the  centre  as  possible,  so  as  to  allow  for  overlap  or 
bond  of  at  least  one  th’ird  of  the  length  of  the  stretcher,  the 
stretcher  being  a  stone  the  length  of  which  is  shown  on  the  face 
of  the  wall.  Sometimes  stones  are  found  in  strata  of  the  thick¬ 
ness  requisite  for  this  kind  of  work,  are  easily  quarried,  do  not 
require  an  excessive  amount  of  cutting  and  dressing,  and  con¬ 
sequently  are  well  adapted  to  the  purpose.  Sandstone  and 
granite  are  generally  quarried  in  much  thicker  blocks,  and  are 
therefore  better  suited  to  structures  requiring  thick  courses, 
and  can  be  more  economically  used  in  the  larger  blocks. 
This  class  of  work  is  suitable  for  almost  any  structure,  unless 
■exposed  to  some  kind  of  shock,  as  in  case  of  lighthouses  and 
bridge  piers,  and  presents  a  neat  appearance,  but  is  not  econom¬ 
ical  unless  the  stone  comes  in  the  quarry  in  small  blocks  and 
with  good  natural  beds. 

78.  Ashlar  Masonry.  This  class  of  masonry  stands  at  the 
head  of  the  list,  and  is  used  in  all  important  structures,  such  as 
large  piers  for  bridges,  lighthouses,  breakwaters,  and  large  pub¬ 
lic  and  even  private  buildings.  Granite  is  used  for  the  most 
important  structures  regardless  of  cost,  but  limestone  or  sand¬ 
stone  are  used  when  cost  enters  as  an  important  factor.  The 
strength  of  this  class  of  masonry  arises  from  the  large  size  of 
the  blocks  used,  the  care  taken  in  cutting  and  dressing  the 
stone,  the  care  taken  in  building  the  structure,  and  the  extent 
of  the  bond  obtainable,  both  longitudinally  and  transversely. 
It  is  laid  in  regular  courses,  of  thicknesses  varying  from  1  to  3 
feet.  The  length  of  stones  from  1  to  4  times  the  thickness  and 
breadth  from  1  to  2  times  the  thickness,  and  with  a  bond 
from  1  to  1 '2-  times  the  thickness.  The  side  and  bed  joints  are 
dressed  to  plane  surfaces  and  at  right  angles  to  each  other ;  it 
is  not  desirable  that  these  should  be  perfectly  smooth  surfaces, 
but  should  present  a  series  of  shallow  ridges  and  hollows, 
such  as  would  naturally  result  from  finishing  with  a  pointing 
tool.  They  should  be  nearly  true  throughout  the  surface  to 


42 


A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


a  straight-edge,  rather  concave  than  convex  towards  the  centre 
of  the  surface.  There  is  little  danger  of  stone-cutters  leaving 
the  stone  convex  on  the  surface,  as  it  is  difficult  to  set  such  a 
stone,  and  tends  to  leave  large  open  joints  on  the  face.  The 
danger,  however,  is  of  cutting  the  face  concave,  thereby  insuring 
a  thin  and  neat  joint  on  the  face.  The  danger  here  is  of  throw¬ 
ing  the  pressure  on  the  edges  of  the  stone,  causing  the  edges 
to  chip  and  spawl  off,  thereby  defacing  the  face  of  the  work. 
If  resulting  in  no  other  harm,  this  effect  can  be  seen  on  the 
face  of  many  structures. 

79.  Ashlar  masonry,  however,  in  large  piers  is  only  used 
on  the  two  faces  and  two  ends,  leaving  a  hollow  centre  space ; 
this  must  be  filled  up  with  something.  This  filling,  whatever  it 
is,  is  called  “backing,”  and  depends  to  a  large  extent  on  the 
whim  of  the  chief  engineer.  Some  engineers  say  ordinary  rubble 
is  good  enough,  some  say  concrete  ;  some  say  large  stones  of 
the  same  thickness  as  the  face  stone,  only  leaving  small  space 
of  from  6  to  12  inches  to  be  filled  with  rubble  or  spawls.  Few 
however,  require  the  backing  stones  to  be  dressed  as  closely  as 
the  face  stones,  but  they  should  be  brought  to  a  good  average 
even  surface  on  the  beds,  though  some  require  the  backing 
stones  to  be  dressed  as  true  as  the  face  stones.  This  latter 
may  be  best,  but  if  the  other  is  good  enough,  why  go  to  the 
greatly  increased  cost.  The  only  reason  that  we  can  build 
ashlar  masonry  at  the  prices  now  existing  is  based  upon  the 
rough  backing  being  used,  as  the  profits  are  drawn  almost 
entirely  from  this  source.  (See  Plate  II,  Figs.  1  and  2.) 

80.  The  joints  in  ashlar  masonry  to  be  filled  with  mortar 
vary  from  ■§•  to  £  inch  in  thickness  on  the  face.  In  actual 
practice,  except  in  some  special  cases,  the  larger  limit  is  prob¬ 
ably  reached  in  most  cases;  there  is  no  need  of  exceeding 
this  limit. 

81.  Assuming  that  the  face  stones  have  been  laid  with  the 
proper  proportions  of  headers  and  stretchers,  how  shall  the 
enclosed  space  be  filled  ?  The  writer  would  fill  with  large 
backing  stones  of  the  same  thickness  as  the  face  stones,  filling 
the  small  vacant  spaces  with  spawls.  A  bad  habit  of  masons  in 


MASONRY. 


43 


this  filling  is  to  put  down  a  pile  of  small  stones,  then  smear  a 
little  mortar  over  the  top.  This  should  not  be  allowed.  A  thick 
bed  of  mortar  should  first  be  thrown  in,  and  the  small  stones 
pressed  and  rammed  into  the  mortar,  then  another  layer  of 
mortar  and  stones  pressed  in,  and  so  on.  This  insures  solid  work, 
and  is  as  easily  done,  if  not  more  so  than  the  other.  The  spaces 
need  not  exceed  6  inches  on  an  average.  The  backing  stone 
should  be  laid  so  as  to  break  joints  both  longitudinally  and 
transversely.  (See  plan  of  pier,  Plate  II,  Fig.  i.) 

82.  The  practice  with  some  engineers,  after  laying  the 
large  backing  stone  in  place,  taking  care  in  all  cases  to  break 
the  joints  in  both  directions,  so  as  to  bond  the  entire  wall  both 
longitudinally  and  transversely,  is  to  fill  the  vacant  spaces  with 
broken  stone  of  varying  sizes,  and  then  “  grout  ”  the  work, 
that  is,  pour  liquid  mortar  into  these  places  until  they  are 
filled,  first  pouring  in  a  liberal  quantity  of  water;  when  filled 
with  mortar  the  water  will  rise  to  the  surface.  The  trouble  is 
that  under  these  conditions  the  cement  and  sand  will  to  a  large 
extent  separate,  the  cement  rising  to  the  top,  thus  forming  a 
series  of  layers  of  sand  with  little  cement  and  of  cement  with 
little  sand,  as  the  sand  will  invariably  sink  to  the  bottom.  This 
at  least  is  the  writer’s  experience.  Others  claim  that  it  is  best 
and  insures  a  solid  wall.  It  is  largely  practiced. 

83.  The  second-best  method  is  to  fill  the  entire  space 
between  the  face  stones  with  good  concrete,  with  headers 
reaching  well  back  into  the  wall  and  some  backing  stone  over¬ 
lapping  the  tails  of  the  headers  from  opposite  faces.  It  has 
always  been  a  puzzle  to  the  writer  why  this  plan  is  not  more 
generally  followed  :  it  would  certainly  insure  a  solid  strong  wall, 
is  more  rapidly  put  in  and  probably  more  economical  than  the 
first  plan,  but  some  prejudice  exists  against  it.  (See  left  half 
of  plan,  Fig.  2,  Plate  II.) 

84.  Lastly,  to  fill  the  vacant  space  between  the  face  stones 
with  rubble.  This  can  be  done  either  by  carefully  bedding 
the  larger  stones  in  mortar,  and  filling  in  between  these  with 
smaller  stones  and  spawls  well  pressed  in  the  mortar,  or  by 
simply  throwing  large  and  small  stones  in  the  vacant  space 


44  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 

until  filled,  then  pouring  grout  over  the  entire  space  until 
all  interstices  are  filled  with  mortar,  as  above  directed.  This 
method  is  doubtless  less  costly  than  either  of  the  other  two.  It 
may  be  good  enough,  but  for  some,  no  doubt  good,  reasons  is 
rarely  adopted  for  important  works.  (See  right-hand  half  of 
Fig.  2,  Plate  II). 

85.  In  whatever  manner  the  backing  is  constructed,  the  wall 
of  the  pier  is  carried  up  from  course  to  course,  each  course 
being  entirely  completed  before  beginning  another  course,  as 
it  is  a  bad  plan  to  build  a  part  of  several  courses  and  leave  a 
series  of  steps,  and  then  build  up  the  rest  of  the  pier  bonding 
on  the  older  work,  which  can  rarely  be  done  as  well  as  in  com¬ 
pleting  entirely  each  course. 

86.  The  neat  work  commences  at  or  a  little  below  the  sur¬ 
face  of  the  ground  or  water,  on  top  of  the  footing-courses 
which  was  called  the  foundation,  and  generally  diminishes  in 
size  gradually  to  the  top  of  the  wall.  This  gradual  decrease  in 
length  and  thickness  is  called  the  batter,  and  is  generally  at 
the  rate  of  -J  inch  to  the  foot  all  round,  or  in  other  words 
diminishes  in  length  and  breadth  1  inch  for  each  vertical 
foot  from  bottom  to  top.  The  bottom  dimensions  are  deter¬ 
mined  from  the  top  dimensions,  which  are  fixed  according  to 
the  purpose  for  which  the  structure  is  intended.  In  case 
of  piers  this  is  fixed  by  the  bridge  companies  who  build  the 
iron  work  or  superstructure,  and  adding  1  inch  for  each  ver¬ 
tical  foot  of  height  gives  the  dimensions  for  the  neat  work  at 
the  bottom.  The  spread  of  the  footing-courses  is  determined 
arbitrarily,  but  generally  arranged  so  as  to  give  from  2  ft.  to 
one  half  of  the  bottom  width  of  the  neat  work  on  each  side, 
the  projection  of  each  course  generally  being  from  6  in.  to  9 
in.,  or  even  12  in.  The  footing-courses  generally  increase  down¬ 
wards  by  offsets  or  steps. 

87.  The  appearance  of  the  stone  on  the  face  of  the  work 
has  nothing  to  do  with  the  classification  of  the  masonry,  this 
depending  entirely  upon  the  size  and  shape  and  the  manner 
of  dressing  the  beds  and  the  joints  of  the  stones.  As  to  the 
appearance  on  the  face,  whether  dressed  smooth,  as  in  the 


MA  SO  NR  Y. 


45 


finest  of  masonry,  such  as  large  public  buildings,  lighthouses, 
etc.,  or  with-chisel  drafts  from  I  to  inches  cut  all  round 
the  edges  of  the  stone,  the  remaining  portion  being  left  with 
quarry  or  rock  faces,  or  whether  a  simple  pitch-line  is  cut 
around  the  edges  of  the  stones,  that  is,  simply  cut  to  a  sharp, 
straight,  well-defined  line,  and  the  entire  face  left  rough,  except 
that  projections  over  4  or  5  inches  are  knocked  off, — none  of 
these  conditions  affect  the  strength  or  durability  of  the  struc¬ 
ture.  The  chisel-draft  aids  in  setting  the  stones  true,  the  one 
above  the  other,  so  as  to  avoid  slight  projections,  and  enables 
the  mason  to  keep  a  regular  and  uniform  batter. 

88.  A  good  pitch-line  fully  meets  these  conditions.  It  is 
considerably  more  economical,  and  in  large  masses  of  masonry 
permits  a  better  and  more  appropriate  appearance  than  the  two 
first  methods.  For  architectural  effect,  as  well  as  to  prevent  a 
continuous  flow  of  rain-water  down  the  face  of  the  pier,  at 
some  suitable  point  a  string-course  is  built  in  the  wall ;  this 
consists  of  broad  stones  well  bonded  into  the  Avail  and  pro¬ 
jecting  from  6  to  9  inches  from  it  all  around,  with  a  wash 
cut  on  the  projecting  portion,  that  is,  cut  on  a  gentle  slope 
downwards.  At  the  top  of  the  wall  is  placed  a  course  of  large 
stones  projecting  from  6  to  9  inches  all  round  the  wall ;  a 
wash  is  also  cut  on  the  projecting  portion  :  this  is  called  the 
coping,  the  object  of  which  is  to  give  a  neat  finish  to  the  top 
of  the  pier,  to  protect  the  smaller  stones  and  rougher  work 
below,  and  at  the  same  time  to  distribute  over  a  large  surface 
the  heavy  concentrated  weight  above.  These  stones  are 
dressed  perfectly  true  and  square  on  all  sides,  and  laid  with  as 
close  joints  as  practicable,  these  joints  being  entirely  filled 
with  a  thin  grout.  The  stones,  owing  to  their  exposed  posi¬ 
tion,  are  generally  fastened  to  each  other  by  iron  cramps,  or 
fastened  to  the  masonry  below  by  long  iron  bolts,  placed  in 
holes  drilled  for  the  purpose  and  fastened  in  place  by  pouring 
in  the  holes  after  the  bolt  is  in  place  either  melted  sulphur, 
melted  lead,  or  cement  grout.  On  top  of  this  coping  another 
coping-course  is  sometimes  laid,  and  then  large  thick  stones  of 
some  hard  material  are  placed  (that  is,  in  case  of  bridge  piers), 


4(r>  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 

from  each  of  which  springs  an  end  post  of  the  bridge  with  its 
pressure  concentrated  on  this  stone.  This  large  stone  is  called 
the  bridge  seat  or  raising  stone,  and  distributes  the  pressure 
over  three  or  four  coping-stones  below,  but  otherwise  is  simply 
a  matter  of  convenience,  and  is  often  entirely  omitted. 

89.  The  appearance  of  a  wall  of  masonry,  on  its  face,  does 
not  necessarily  determine  the  character  of  the  masonry.  It 
may  look  well,  and  seemingly  in  accordance  with  the  specifica¬ 
tions  ;  the  headers  may  only  be  blocks,  or  “  bobtails,”  as  they 
are  called ;  stretchers  may  have  less  breadth  than  thickness, 
and  the  interior  bonds  may  be  poor  ;  that  this  not  only  may  be 
the  case  and  often  is,  cannot  be  doubted  or  denied.  The  inte¬ 
rior  condition  is  only  fully  known  by  the  builder,  the  most  rigid 
inspector  cannot  ordinarily  prevent  it,  but  mutual  confidence 
and  reasonableness  between  the  two  will  largely  do  so.  It  is 
not  unusual  to  specify  that  the  headers  and  stretchers  should 
not  be  less  than  3  feet  long,  and  likewise  that  they  should  not 
be  more  than  6  feet  long.  As  to  the  length  of  the  headers,  it 
would  seem  better  to  proportion  this  to  the  thickness  of  the 
wall  at  that  point.  Walls  are  generally  built  in  courses  of 
varying  thickness,  and  generally  decreasing  from  bottom  to 
top,  the  thicker  courses  being  at  the  bottom,  and  the  width  of 
the  piers  varies  from  15  feet  to  20  feet  at  bottom  to  6  feet  to 
12  feet  at  top  ;  and  with  the  limitation  that  a  header  should 
never  be  less  than  3  feet  long,  the  headers  should  generally 
vary,  from  6  feet  at  bottom  to  3  feet  at  top  of  the  wall.  A 
3-foot  header  in  a  course  from  2  to  3  feet  thick  would  practi¬ 
cally  be  of  no  use,  but  in  a  high  pier  it  would  be  difficult  to 
build  it  without  securing  a  good  bond  throughout. 

Article  VIII. 

ORNAMENTATION. 

90.  Although  ornamentation  is  of  secondary  consideration 
In  large  massive  structures  such  as  bridge  piers,  yet  a  good 
effect  can  be  produced  by  a  simple  string  or  belt  course  at 
some  suitable  point  in  its  height.  This  is,  however,  seldom 


ORNAMENTA  TION. 


47 


used  with  square  ended  piers,  but  with  rounded  or  pointed  ends 
it  is  usual.  The  curved  or  pointed  end  is  generally  built  to  a 
point  a  little  above  high-water,  and  the  upper  part  is  completed 
to  the  top  of  the  pier  with  square  ends,  which  is  then  finished 
off  with  a  suitable  coping.  The  string  or  projecting  course  is 
usually  placed  at  the  dividing  line  between  the  rounded  and 
square  end  of  the  pier,  and  a  low  conical-shaped  finish  on  top 
of  the  belt-course  makes  this  passing  from  the  one  to  the  other 
pleasing  to  the  eye.  To  make  the  templets,  a  platform  of 
wood  is  made,  a  centre  point  fixed  ;  a  round  iron  pin  is  then 
driven  at  that  point,  and  a  straight-edge  laid  flat,  with  a  small 
hole  near  one  end,  can  be  made  to  revolve  around  this  as  a 
centre :  another  hole  is  bored  at  a  distance  from  the  first  equal 
to  one  half  the  width  of  the  pier  at  the  bottom,  and  a  spike  or 
pencil  fastened  in  this  will  describe  a  proper  circumference  on 
the  platform,  from  which  the  templets  can  be  cut.  A  pencil  in 
other  holes  I,  or  i-J  inches  from  each  other,  according  to  the 
batter  and  the  thickness  of  the  course,  will  describe  the  proper 
circle  for  the  different  courses.  (For  circular  ends  see  Plate  II, 
Fig.  i.) 

91.  For  elliptical  ends  they  may  either  be  a  part  of  one 
ellipse  whose  conjugate  axis  is  the  width  of  the  pier,  or  some 
portion  of  the  semi-ellipse,  the  double  ordinate  or  base  of 
which  is  the  width  of  the  pier,  in  which  case  the  foci  are 
marked  on  the  board  at  a  distance  apart  to  be  determined  by 
the  shape  of  the  point  and  the  length  of  the  rounded  end  re¬ 
quired,  which  will  be  largely  a  matter  of  taste.  At  the  foci 
drive  spikes,  and  with  a  string  equal  in  length  to  the  trans¬ 
verse  diameter  of  the  ellipse,  its  ends  fastened  to  the  spikes, 
then  with  a  spike  or  pencil,  drawing  the  string  tight  and  keeping 
it  taut  all  the  time,  the  pencil  will  describe  the  ellipse  ;  select¬ 
ing  a  point  on  the  curve,  whose  double  ordinate  is  equal  to  the 
width  of  the  pier,  this  will  be  the  base  of  the  templet,- which 
then  must  be  cut  to  conform  to  the  curve  of  the  vertex  of  the 
ellipse.  Sometimes  the  ends  are  formed  by  parts  of  two  in¬ 
tersecting  ellipses,  which  must  be  similarly  constructed  on  the 
platform.  The  sizes  of  these,  as  said  above,  are  mere  matters 


48  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


of  taste :  the  length  from  the  body  of  the  pier  to  the  point 
is  generally  about  equal  to  the  one-half  width  of  the  pier.  For 
triangular  ends,  the  sides  are  plane,  the  base  is  equal  to  the 
width  of  the  pier,  and  the  altitude  equal  to  one  half  that  width. 
(For  elliptical  and  triangular  ends  see  Fig.  2.) 

92.  All  of  these  ends,  called  starlings  or  cutwaters,  are 
dressed  on  the  exposed  surfaces  either  smooth  or  approxi¬ 
mately  so,  and  are  generally  carried  up  with  the  regular  batter 
of  ^  inch  to  the  vertical  foot,  and  are  placed  generally  at  both 
ends  of  the  pier  for  symmetry,  but  they  are  only  necessary  at 
the  up-stream  end.  These  portions  of  the  pier  are  not  consid¬ 
ered  as  bearing  any  part  of  the  weight  of  the  structure,  but  to 
split  and  turn  aside  drift  and  ice,  or  in  some  cases  to  prevent 
any  scouring  tendency  by  offering  less  resistance  to  the  cur¬ 
rent.  They  should,  however,  be  carefully  bonded  into  the  pier, 
and  in  some  cases  it  is  best  to  bolt  them  to  each  other.  In 
some  cases  looks  are  thrown  aside,  and  a  well-defined  cut¬ 
water  is  placed  at  the  up-stream  end  of  the  pier,  the  lower 
being  square. 

93.  A  strictly  called  cut-water  is  built  on  the  up-stream  end 
alone.  This  is  used  where  the  piers  are  very  high  and  thick, 
and  large  masses  of  ice  have  to  be  dealt  with.  This  is 
built  from  a  little  distance  below  low-water  to  a  point  a  little 
above  high-water — generally  not  over  12  or  20  feet.  It 
may  be  described  as  an  oblique  pyramid  projecting  from  the 
body  of  the  pier,  the  up-stream  edge  sloping  towards  the 
pier  at  an  angle  of  forty-five  degrees.  Near  this  edge  the 
sides  are  dressed  smooth,  forming  a  sloping  prism,  whose 
base  is  a  triangle,  the  base  of  which  triangle  is  the  width 
of  the  pier  and  the  altitude  one  half  to  one  time  that  width. 
The  remaining  portion  of  the  cutwater  is  solid  masonry,  of  the 
same  width  of  the  pier;  the  end  stones  should  be  thoroughly 
fastened  to  each  other  by  iron  bolts  and  cramps.  This  form 
of  starling  will  split  and  break  immense  sheets  of  ice  of  great 
thickness.  (See  Plate  XIX,  Figs.  1,  2,  and  3). 


ICE  AND  WIND  PRESSURE . 


49 


Art.  IX. 

ICE  AND  WIND  PRESSURE. 

94.  In  piers  of  bridges,  under  normal  conditions,  the  press¬ 
ures  are  vertical,  and  as  the  centre  of  pressure  is  in  the  centre 
of  the  figure  of  the  base,  the  pressures  are  uniformly  distrib¬ 
uted  ;  hence  there  is  no  danger  of  sliding,  as  the  bed  joints  are 
horizontal  or  perpendicular  to  the  pressure,  and  no  danger  of 
overturning,  as  the  pressures  are  all  vertical,  and  the  piers  only 
have  to  be  strong  enough  to  resist  crushing.  Rut  under  some 
circumstances  they  are  subjected  to  unusual  forces,  such  as  high 
winds,  which  not  only  act  directly  against  the  pier,  but  upon 
the  superstructure  and  upon  the  train  that  may  be  passing 
over  the  bridge ;  also  from  the  current  acting  upon  large  fields 
of  ice,  which  sometimes  gorge  or  bank  up  to  the  depth  of  many 
feet ;  and  when  a  solid  mass  bridging  the  river  exists,  each 
pier  is  supposed  to  carry  a  pressure  due  to  a  mass  the  depth 
of  the  ice  by  length  of  a  half  span  on  either  side,  and  from 
the  wind  pressure  that  is  exerted  on  the  truss  and  train  for 
the  length  of  a  half-span  on  either  side.  Both  of  these  pres¬ 
sures  are  unknown,  but  by  assuming  values  for  these  based 
upon  such  data  as  we  have,  the  problem  is  a  very  simple 
one.  Trautwine  states  that  the  pressure  per  square  foot 
exerted  by  the  wind  upon  a  surface  exposed  at  right  angles  to 
its  direction  is  equal  to  the  square  of  the  velocity  in  miles  per 
hour  multiplied  by  the  area  of  the  surface  and  divided  by  200, 
viz.,  V"1  A  A-  200  =  pressure  in  pounds  per  square  foot,  at  40 
miles  per  hour,  and  A  —  1  sq.  ft.,  the  pressure  per  square  foot 
is  equal  to  8  lbs.;  and  for  V  =  100  miles  per  hour,  the  press¬ 
ure  per  square  foot  equal  to  50  lbs.,  and  so  on.  A  velocity  of 
100  miles  per  hour  is  a  hurricane.  The  pressure  from  the  field 
of  ice  or  gorge  is  certainly  unknown.  The  ice  in  the  Susque¬ 
hanna  River  at  Havre  de  Grace  often  freezes  to  the  thickness 
of  2  feet.  The  writer  has  seen  it  from  15  to  20  inches  thick  in 
a  solid  sheet  from  shore  to  shore.  It  moves  in  this  solid  mass  6 


50 


A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


or  io  feet  at  a  time,  and  repeadedly,  before  it  breaks  up.  The 
cutwaters  on  these  piers  would  split  it  from  50  to  100  feet 
above  the  pier,  the  mass  rising  up  over  the  cutwater ;  and 
while  this  was  going  on  the  broken  ice  from  a  distance 
above  would  be  rising  on  top  and  sinking  under  this  im¬ 
mense  sheet  of  ice  to  unknown  depths.  These  facts  are 
mentioned  to  show  the  enormous  and  unknown  pressure  to 
which  these  piers  are  subjected  nearly  every  winter.  This 
probably  represents  as  great  a  pressure  from  this  source  as 
is  likely  to  occur  anywhere,  and  as  showing  that  piers  as 
built  from  necessity  are  sufficiently  large  and  heavy  to  resist 
all  these  outside  forces.  Some  authorities  give  about  double 
the  pressure  from  the  wind  as  above  given,  but  by  assuming 
50  lbs.  per  square  foot  of  exposed  surface  it  is  doubtless  on  the 
safe  side. 

95.  As  bridge  trusses  are  open  work,  it  is  generally  as¬ 
sumed  that  the  exposed  surface  is  double  the  area  of  one 
truss  for  an  unloaded  truss,  and  on  a  loaded  structure  30  lbs. 
wind  pressure  per  square  foot  of  total  truss  surface,  and  in 
addition  an  equal  amount  per  square  foot  of  train  surface,  the 
latter  treated  as  a  moving  load  ;  and  as  the  good  practice,  though 
far  from  uniform,  we  may  take  truss  and  train  as  exposing 
together  20  square  feet  per  foot  of  length,  equivalent  to 
600  lbs.  pressure  per  foot  of  length,  and  for  a  pier  carrying 
two  525-ft.  spans,  a  total  pressure  of  315,000  lbs.,  equal  to 
160  tons.  Assuming  that  the  pier  splits  the  ice  for  a  dis¬ 
tance  of  50  feet  above  the  pier,  the  ice  being  2  feet  thick, 
and  assuming  the  resistance  to  be  10  tons  per  square  foot,  we 
would  have  50  X  2  X  10  =  IOOO  tons.  Moment  of  overturning 
due  to  wind  pressure  =  160  tons  multiplied  by  lever  arm 
(height  of  pier  plus  one  half  of  truss,  equal  to  100  plus  30  equal 
to  130  feet)  =  to  20,800  ft. -tons.  The  ice-pressure  at  the 
Susquehanna  was  doubtless  the  greatest  at  a  rather  low  stage 
of  water,  but  for  safety  we  can  assume  the  lever  arm  to  be  20 
feet.  We  have  1000  X  20  equal  to  20,000  ft.-tons  ;  but  double 
this  and  make  it  40,000  ft.-tons,  there  results  then  total  over¬ 
turning  moment  equal  to  6o,8co  ft.-tons.  The  weight  of  one 


ICE  AND  WIND  PRESSURE. 


51 


of  the  piers  would  equal  4350  tons,  and  the  weight  of  one  half 
span  on  either  side,  or  one  entire  span,  say  700  tons,  and  weight 
of  empty  train  and  cars,  200  tons,  or  total  5250  tons,  which 
multiplied  by  one  half  the  length  of  pier,  equal  to  20  ft.,  then 
the  moment  of  resistance  to  overturning,  would  be  105,000-ft.- 
tons,  or  a  factor-of-safety  of  about  if.  These  results  are  based 
upon  the  most  unparalleled  conditions,  by  increasing  the  ten¬ 
dency  to  overturn  far  beyond  that  which  is  likely  to  arise,  as 
probably  extreme  pressures  are  assumed,  and  these  supposed 
to  act  together,  which  would  rarely  occur,  and  the  train  is 
supposed  empty  in  addition.  The  increased  crushing  pressure 
is  not  worth  considering.  The  stability  of  pivot  piers  is  cer¬ 
tainly  equal  to  if  not  greater  than  that  of  the  corresponding  rest 
piers,  and  in  addition  they  are  protected  from  ice  pressure  by 
guard  piers  especially  constructed  and  entirely  separate  from 
the  pier  itself,  which  serve  also  as  rest  and  protection  piers 
to  the  ends  of  the  draw-span  when  open.  These  guard  piers 
are  built  of  masonry,  iron,  or  timber.  They  are  also  required 
above  and  below  both  pivot  and  rest  piers  in  some  cases,  and 
when  built  of  timber  or  masonry  faced  with  timber  act  as 
guiding  dikes  for  the  passage  of  vessels  and  steamboats; 
when  detached  masonry  guard  piers  are  used,  a  floating  crib  or 
strong  box  is  built  between  the  guard  piers  and  the  pivot  pier, 
which  rises  or  falls  with  the  water,  being-connected  with  the 
pier  by  properly  arranged  sliding  surfaces. 

96.  The  top  dimensions  are  fixed  by  the  bridge  companies 
so  as  to  allow  ample  room  for  the  superstructure,  but  in  gen¬ 
eral  for  piers  the  dimensions  vary  according  to  the  length  of 
span,  from  6  feet  by  20  feet  to  12  feet  by  40  feet,  and  the 
bottom  dimensions  of  the  neat  work  are  fixed  generally  by 
allowing  one  inch  for  each  vertical  foot,  but  sometimes  by 
abrupt  enlargements  in  addition.  Pivot  piers  are  generally 
round,  and  vary  at  top  from  20  feet  to  30  feet,  so  as  to  allow 
ample  margin  for  the  turntable  arrangements. 


52 


A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


Article  X. 

RETAINING-W  ALLS. 

97.  Ordinary  earth  will  not  stand  for  any  length  of  time 
with  a  vertical  face,  but  will  generally  assume  a  slope  more  or 
less  steep,  according  to  the  nature  of  the  material,  the  angle 
of  this  slope  is  determined  by  the  adhesion  between  the  par¬ 
ticles  composing  it  and  the  friction  between  these  particles  or 
grains.  The  adhesion  between  the  grains  is  destroyed  by  the 
disintegrating  effects  of  air  and  moisture,  therefore  we  may 
say  that  friction  alone  determines  the  angle  of  the  slope.  The 
angle  which  this  slope  finally  assumes,  measured  from  the  hori¬ 
zon,  is  called  the  “  angle  of  repose  the  slope  itself  is  called 
the  “  natural  slope.”  When  an  earth  embankment  either 
reaches  this  slope  from  natural  causes  or  is  built  with  this 
slope,  its  stability  is  insured.  The  effects  of  running  water, 
from  rain  or  other  causes,  will  wash  it  in  ruts  and  gullies,  but 
this  can  be  provided  against  by  sodding,  paving,  or  good  drain¬ 
age. 

98.  It  is  often  necessary,  however,  to  maintain  a  vertical 
face,  as  behind  abutments  when  the  approaches  of  the  bridge 
are  built  of  earth,  as  well  as  in  other  similar  cases.  It 
then  becomes  necessary  to  build  a  wall  of  some  kind,  called  a 
retaining-wall,  or  in  case  of  supporting  the  pressure  of  water,  a 
reservoir  wall.  The  principles  of  stability  of  these  walls  are 
the  same. 

99.  The  resulting  force  acting  on  a  retaining-wall,  or  the 
abutment  of  arches,  is  always  inclined  to  the  vertical,  more  or 
less,  depending  upon  the  relative  intensity  of  the  weight  of  the 
wall  acting  vertically  through  the  centre  of  gravity  of  its  mass, 
and  the  intensity  of  the  pressure  of  the  earth  on  the  wall, 
together  with  its  direction.  This  obliquity  of  the  resultant 
force  causes  two  tendencies:  the  one  is  to  cause  the  wall  to 
slide  upon  the  foundation-bed  or  upon  some  bed  of  the  wall 
itself,  the  other  is  to  overturn  the  wall  bodily  around  some 


RE  TA INING-  WALLS. 


53 


axial  line.  The  first  tendency  can  easily  be  provided  against 
by  so  arranging  the  foundation-bed,  or  some  courses  of  the 
masonry,  that  their  direction  may  be  perpendicular  to  the 
direction  of  the  force  or  pressure.  A  horizontal  foundation- 
bed  will  generally  give  security  against  this  tendency,  unless 
the  wall  rests  upon  slippery  and  inclined  layers  of  earth. 

100.  The  tendency  to  overturn  can  only  be  resisted  by  suffi¬ 
cient  thickness  and  weight  of  wall  to  fulfil  the  two  following 
conditions : 

1st.  That  the  direction  of  the  resultant  pressure  must  not 
pierce  the  foundation-bed  further  from  its  geometrical  centre 
than  a  certain  limit,  which  may  be  taken  at  three  eighths  of 
the  thickness.  This  point  is  called  the  “  centre  of  pressure.” 
This  mode  of  stating  the  condition  is  a  substitute  for  a  factor- 
of-safety,  as  the  actual  point  of  overturning  would  only  be 
reached  when  the  direction  of  the  resultant  pressure  passed 
through  the  outer  edge  of  the  wall. 

2d.  That  the  moment  of  weight  of  the  wall  with  respect 
to  an  axis  passing  through  the  centre  of  pressure  shall  be  at 
least  equal  to  or  greater  than  the  moment  of  the  outside  press¬ 
ure  on  the  wall  in  respect  to  the  same.  This  axis  is  taken  as 
passing  through  the  centre  of  pressure  rather  than  through  or 
along  the  outer  edge  of  the  masonry,  for  reasons  of  safety,  as 
above  stated,  as  the  effect  is  to  reduce  the  actual  moment  of 
the  weight  and  to  increase  the  moment  of  pressure. 

101.  This  subject  has  been  theorized  and  experimented  on 
perhaps  as  much  as  any  other  engineering  problem  except  that 
of  arches.  Formulae  are  conflicting,  owing  to  the  uncertainty 
and  variety  of  conditions  actually  existing.  Many  are  the 
results  of  miniature  experiments.  Mr.  Rankine  evolves  a  for¬ 
mula  purely  from  theoretical  or  supposed  conditions, — all,  no 
doubt,  approximating  the  truth. 

102.  The  requisite  thickness  of  the  wall  is  a  certain  fraction 
of  the  height.  The  practical  result,  however,  obtained  is  that 
at  any  point  of  the  wall,  from  the  top  to  the  bottom,  the 
thickness  of  the  wall  must  be  not  less  than  one  third  of  the 
vertical  height  from  the  surface  of  the  ground  to  that  point,  and 


54 


A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


need  not  in  general  be  more  than  one  half  the  height.  Two 
fifths  of  the  height  may  generally  be  taken  as  a  safe  thickness, 
all  depending  upon  the  nature  of  the  material  resting  against  the 
wall.  If  this  material  is  in  the  nature  of  a  fluid,  such  as  water, 
quicksand,  and  the  like,  a  greater  thickness  may  be  required — 
even  equal  to  the  height.  If  there  is  danger  of  the  material 
being  converted  into  a  flowing  mass  by  the  presence  of  water, 
a  good  plan  is  to  place  a  vertical  (or  nearly  so)  layer  of  broken 
stone  or  gravel  between  the  material  and  the  wall.  This  will 
serve  to  carry  off  the  water,  small  holes  being  left  through  the 
wall  to  allow  the  water  to  escape.  Retaining-walls  sometimes 
bulge  outwards  without  sliding  on  the  foundation-bed  or  over¬ 
turning.  When  such  is  the  case  the  wall  may  be  considered 
in  a  precarious  condition,  but  new  relations  between  the  press¬ 
ures  arising  therefrom  may  result  in  a  condition  of  stability, 
and  the  wall  may  remain  in  its  then  condition  for  a  long  time. 

103.  The  face  of  a  retaining-wall  is  generally  built  of  rough 
ashlar  masonry,  may  be  built  of  block-in-course  or  of  brick, 
for  two  reasons:  1st.  The  main  pressure  is  concentrated 
towards  the  face  of  the  wall,  and  a  better  class  of  masonry  is 
required.  2d.  For  the  sake  of  appearances.  The  back  of  the 
wall  is  generally  of  a  rough  rubble,  composed  of  large  and 
small  stones.  The  face  and  back  should  be  thoroughly  tied 
or  bonded  together,  so  that  the  entire  wall  may  act  together 
in  resisting  the  pressure.  The  face  of  the  wall  is  generally 
built  on  a  batter,  as  in  piers,  but  the  back  is  almost  always 
built  in  a  series  of  steps  of  greater  or  less  rise.  This  increases 
the  stability  of  the  wall,  by  bonding  into  the  material  behind, 
and  having  its  weight  increased  by  the  weight  of  the  natural 
material  resting  upon  it.  Some  additional  stability  can  be 
secured  by  inclining  the  wall  backwards  towards  the  pressure, 
or  the  same  stability  by  this  method  can  be  secured  with  less 
masonry. 

104.  The  face  of  the  wall  is  built,  as  in  case  of  piers,  rest¬ 
ing  on  the  usual  footing-courses,  both  to  distribute  the  pressure 
over  a  larger  surface,  and  at  the  same  time  to  throw  the  centre 
of  pressure  further  inward  from  the  face  of  the  wall. 


RE  TAINING-  WALLS. 


55 


105.  When  a  retaining-wall  is  in  the  nature  of  a  railroad 
abutment,  or  the  abutment  pier  of  arches,  supporting  a  narrow 
embankment  with  the  ordinary  slopes,  which  generally  are  at 
the  rate  of  i|  feet  horizontal  to  each  foot  of  vertical  height, 
it  is  necessary  to  build  at  each  end  wing  walls  constructed  as 
the  face  wall  of  the  abutment,  that  is,  with  ashlar  face  and 
rubble  backing.  These  wings  can  be  built  in  the  prolongation 
of  the  face  wall,  but  decreasing  in  height,  generally  by  a  series 
of  steps,  as  the  slope  of  the  bank  descends  ;  the  total  length  of 
this  wing  at  bottom  would  then  be  equal  to  times  the  total 
height,  the  object  being,  by  following  with  the  masonry  the 
slope  of  the  embankment,  to  prevent  the  earth  from  falling  in¬ 
front  of  the  abutment.  Sometimes  the  wings  run  to  the  front 
and  perpendicular  to  the  main  walls  for  a  length  determined 
by  the  circumstances  of  the  case.  This  plan  is  rarely  used, 
except  in  coming  out  of  a  tunnel,  to  support  the  sides  of  the 
open  excavation  ;  it  of  course  adds  immensely  to  the  stability 
of  a  wall.  Or  the  wings  may  extend,  in  case  of  an  embankment, 
perpendicularly  to  the  rear  for  a  distance  equal  to  from  I  to  1^ 
times  the  horizontal  base  of  the  slop.e  More  commonly  the 
wings  make  an  obtuse  angle  with  the  face  of  the  wall,  depend¬ 
ing  upon  the  circumstances  of  the  case.  This  plan  is  spe¬ 
cially  applicable  to  abutments  on  the  banks  of  watercourses, 
where  from  the  direction  of  the  current  there  would  be  danger 
in  times  of  floods  of  the  water  getting  behind  the  abutments 
and  scouring  out  the  embankment.  The  angle  between  the 
main  wall  and  the  wing  walls  depending  on  the  angle  between 
the  current  of  the  stream  and  the  direction  of  the  embankment, 
and  even  when  the  directions  of  the  stream  and  the  embankment 
are  at  right  angles,  it  has  the  advantage  of  presenting  a  funnel- 
shaped  entrance  and  exit  for  the  water,  thereby  relieving  the 
danger  of  obstructing  the  free  flow  of  the  stream.  The  wings 
adding  considerable  stability  to  the  main  walls,  these  may  not 
be  as  thick  as  required  in  isolated  walls,  resulting  in  a  small 
saving  of  masonry  on  each  abutment ;  and  on  a  long  line  of 
road  a  little  saved  here  and  there  amounts  to  an  important 
item  of  cost.  Main  and  wing  walls  should  be  finished  with 


56  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 

good  large  coping-stones,  but  these  need  not  be  cut  or  dressed 
as  neatly  as  is  the  custom  on  piers.  . 

106.  In  designing  retaining-walls  for  the  abutments  of  a 
bridge  the  steps  on  the  back  are  so  arranged  that  on  the  top  of 
any  step  the  thickness  of  the  wall  should  be  from  ^  to  £  the 
height  from  the  surface  to  that  point;  this  is  then  carried  up 
vertically  for  a  certain  distance,  then  another  step  is  made,  and 
so  on.  The  back  of  the  wall  may  be  as  rough  as  the  builder 
pleases,  provided  the  minimum  thickness  is  maintained,  and  to 
avoid  unnecessary  care  the  builder  always  makes  it  thicker  than 
required.  On  the  front  a  bridge  seat  of  from  3  to  5  feet  in  width 
must  be  provided  for  the  end  rest  of  the  bridge,  and  back  of  this 
a  wall,  called  a  breast  wall,  must  be  built  up  to  the  under  side  of 
the  crossties  on  the  bridge  ;  this  is  made  from  2  to  2|-  feet  thick 
and  from  2  to  4  feet  high,  depending  upon  the  length  of  the 
span  and  form  of  truss  used,  this  information  is  obtained  from 
the  bridge  company.  The  bottom  dimensions  are  determined 
from  this  data,  as  in  case  of  piers.  In  very  high  walls  the 
centre  of  pressure  on  the  foundation  may  vary  materially  from 
the  centre  of  figure  of  the  base,  and  care  must  be  taken  to 
keep  it  within  the  limits  above  prescribed  in  order  to  avoid  too 
great  unit  of  pressure  on  the  base ;  this  can  be  done  by  spread¬ 
ing  the  base  with  concrete  and  offset  courses. 

107.  In  order  to  prevent  any  tendency  to  slide,  the  condi¬ 
tion  of  frictional  stability  must  be  fulfilled,  which  is  that  the 
direction  of  the  resultant  pressure  must  not  make  with  the 
normal  to  any  horizontal  plane  from  bottom  to  top  an  angle 
greater  than  the  angle  of  repose, — that  is,  than  the  angle  at 
which  the  upper  portion  would  slide  on  the  lower.  There  is 
practically  no  danger  of  the  sliding  of  one  course  of  masonry 
on  another,  but  the  wall  may  slide  as  a  whole  upon  its  base ; 
but  in  either  event  this  tendency  can  be  prevented  by  inclining 
the  plane  of  the  bed-joints  so  as  to  be  nearly  perpendicular  to 
the  direction  of  the  resultant  pressure,  or  the  foundation  bed 
can  be  cut  into  the  form  of  steps.  Good  judgment  can  alone 
determine  when  these  things  are  necessary.  Cases  have  arisen 
when  it  was  necessary  to  anchor  the  wall  by  the  use  of  long 


RE  TA INING-  IV A  LLS. 


57 


rods  passing  through  the  wall  and  fastened  to  iron  plates  or 
timber  walls  embedded  in  the  ground  some  distance  behind  the 
wall,  or  by  inclined  struts  in  front  resting  at  one  end  against 
the  wall,  and  the  other  against  walls  embedded  in  the  ground 
in  front.  These  rods  or  struts  should  generally  rest  against  the 
wall  as  near  the  bottom  as  convenient, — theoretically  at  a  point 
of  the  height  from  the  bottom. 

108,  The  piers  of  a  bridge  on  the  Warrior  River  in  Alabama 
built  on  and  too  near  the  sloping  banks,  without  driving  piles 
for  them  to  rest  on,  had  to  be  held  in  position  by  a  system  of 
strong  struts  as  described  above  ;  and  pneumatic  tubes  were 
sunk  in  an  inclined  position  in  front  of  the  abutment  piers  of 
a  bridge  across  the  Schuylkill  River  in  Philadelphia  to  prevent 
a  continuation  of  a  sliding  discovered  after  completion  of  the 
bridge ;  and  other  instances  could  be  cited,  but  withal  it  is 
rarely  required. 

In  very  high  abutments  large  archways  are  often  left  under 
the  wing  walls,  extending  backward,  for  roadways  as  well  as  for 
economy. 

Article  XI. 

FORMULAE  FOR  THICKNESS. 


109.  The  various  theories  and  resulting  formulae  seem 
either  to  be  based  on  uncertain  or  erroneous  data,  and  without 
undertaking  to  discuss  or  criticise  these  the  writer  will  content 
himself  with  giving  the  geometrical  representations  of  the 
conditions  of  stability  and  the  common  formulae  based  upon 
the  supposed  conditions.  Mr.  Rankine  assumes  that  the  direc¬ 
tion  of  the  pressure  is  parallel  to  the  surface  of  the  ground, 
that  its  intensity  is  uniformly  varying,  and  that  its  amount  is 
represented  by  a  prism  whose  base  is  a  triangle,  sab,  Fig.  4, 
and  whose  length  is  the  length  of  the  wall.  The  triangular  base 
is  constructed  as  follows  :  From  a,  the  bottom  of  the  wall,  draw 


ab  = 


parallel  to  the  surface  of  the  ground  and  also  draw 


the  line  sb  ;  then  area  of  triangle  sab  —  \ab  X  sc  —  also  to  vol- 


58  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


Fig. 4.— Cross-section  of  Retaining-wall.  Prism  of  Earth  Pressure.  Resultant  of 

Earth  Pressure. 

EO  —  surface  of  ground  ; 

0  =  angle  of  slope  of  ground; 

<p'  =  “  “  repose  ; 

sab  =  area  of  base  of  earth  prism  ; 
as  —  x  =  vertical  side  of  sab ; 
sc  —  x  cos  6  =  altitude  of  “ 

p\  cos  0  — 4/cos2  0  —  cos2  cp 

ab  —  x  —  —  x - r— j —  ; 

p  cos  6  1/ cos2  0  —  cos2  4> 

P  —  pressure  of  the  earth  =  ti u\ 

IV  —  weight  of  wall  =  tiwa  ; 
t  —  thickness  of  wall  =  Aa  ; 
or  —  centre  of  gravity  of  triangle  sab  ; 
g,  —  “  “  “  “  wall ; 

2  =  resultant  pressure  ; 
r  —  centre  of  “ 

gxW  =  line  of  action  of  weight  of  wall  ; 
ri  —  qt  —  generally  f  t ; 
ki  =  q,t  may  be  -(—  or  —  ; 

ha  =  }x;  ha  1  =  \x  cos  0;  rxax  —  ay  —  (qt  +  \t)  sin  0. 


RE  TA INING-  W A  LL  S. 


59 


ume  of  earth  prism  unity  in  length  ;  and  if  w'  —  weight  of  unit 


Substituting  values  given  above,  we  have,  for  the  moment  of 
the  pressure  tending  to  overturn  the  wall, 


w' x*  cos  6 


X  ~  X  cos  0  —  (q  -f-  %)t  sin  0).  (i) 


P  x  vr  -- 


The  moment  of  resistance  to  overturning  will  be  the  weight 
of  the  wall  -j-  the  weight  of  earth  resting  on  the  steps  (=  W) 
multiplied  by  its  lever  arm  (=  rP)  =  W.rk  —  W{ri  ±  ki)  — 
W(}t  ±  qxf).  The  quantity  qj  will  be  positive  or  negative  as 
the  line  of  action  of  the  weight  gx  W  is  on  the  opposite  or  the 
same  side  of  the  centre  of  figure  of  the  base  Aa  as  the  centre 
of  pressure ;  in  the  Fig.  i  above  it  is  negative.  The  moment 
of  stability  is  thus  M  —  W(qt  ±  qj),  and  this  must  be  equal  to 
or  greater  than  the  moment  of  the  external  pressure  P  X  vr\ 
q  being  generally  assumed  =  -§. 


X^rX(^  cos  0  —  (q  -f-  £)t  sin  0).  (2) 


The  condition  to  resist  sliding  or  stability  of  friction  is  that 
the  angle  rtxk  <  <p,  the  angle  of  repose  of  masonry  on  masonry, 
<p  varying  from  250  to  30°.  Generally,  all  the  quantities  in 
eq.  2  are  given  except  t,  which  can  then  be  found.  4>  can  be 
measured  or  calculated.  In  Fig.  I, 


0,0,  =  (0,w,  +  tgv,)  X  tang  oJ,o  ; 
c \o2  —  P  cos  6 ;  oxwa  —  P  sin  d ;  txwa  =  W ; 


P  cos  6 


tang  ojj,  = 


W -j-  Psin  0  — 


<  tang  0 


we  have 


6o 


A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


as  the  condition  of  frictional  stability.  For  fluid  pressure 
6  =  o;  cos  6  —  i  ;  sin  6  =  0;  —  =  i. 

P 

Eq.  2  then  becomes 

. (3) 


and 


P 

—  =  tang  <  tang  0 . (4) 


This  whole  theory  from  beginning  to  end  is  beautiful,  and 
if  the  premises  are  true  the  conclusions  are  also.  It  is  analo¬ 
gous  to  the  pressure  of  water,  and  the  formula  is  easily  appli¬ 
cable  to  the  pressure  of  water  by  making  both  6  and  0'  equal 
to  0,  the  formula  then  becomes 


m+& 


W  X 


W  X 
~6~ 


p  cos  6  —  V cos2  6  —  cos2  0' 

as  —  — -  —  —  1 

P  cos  0  - \-  V cos2  0  —  cos2  0' 

in  this  case.  A  practical  example  will  be  given  in  a  subsequent 
paragraph. 

1 10.  The  only  other  formula  that  will  be  mentioned  may 
be  called  Moseley’s.  This  is  based  upon  the  following  condi¬ 
tions,  namely,  when  a  mass  of  earth  is  allowed  to  assume  its 
own  slope  it  will  generally  slide  down  until  the  slope  makes  an 
angle  with  the  horizontal  equal  to  the  angle  of  repose,  then  it 
will  have  its  natural  slope ;  but  if  a  wall  with  a  vertical  face  be 
built  to  prevent  this  sliding,  a  pressure  will  be  exerted  against 
the  wall  by  the  tendency  to  slide.  Experiment  or  theory  or 
both  show,  however,  that  the  weight  of  this  sliding  mass  does 
not  represent  the  maximum  pressure,  but  if  a  plane  be  taken 
bisecting  the  angle  between  the  natural  slope  and  the  back  of 


RE  TAINING-  WALLS. 


61 


the  wall,  the  prism  nearest  the  wall  will  exert  the  maximum  pres¬ 
sure.  The  direction  of  the  pressure  is  taken  to  be  horizontal, 
and  the  point  of  application  of  the  resultant  pressure  on  the  back 
of  the  wall  will  be  at  ^  of  the  height  of  the  wall  from  the  bot¬ 
tom.  Let  AB  (Fig.  5)  be  the  back  of  the  wall  and  vertical,  BC 
the  natural  slope  ;  then  BD  bisecting  the  angle  ABC  will  be  the 
plane  of  rupture,  and  the  prism  ABD  will  produce  the  maxi¬ 
mum  pressure.  Hence,  considering  the  length  of  the  wall 
unity,  the  area  ABD  will  also  be  the  volume.  Area  ABD 
AB  X  AD  D  tang  ABD 


2  2 

w' x1  tang  ABD 


equal  to 

w'x*  tang2  ABD 


and  the  weight  W'  of  the  prism 
and  the  pressure  due  to  this  is  P 


W'  tang  ABD ,  and  the  condition  of  sta¬ 


bility  will  be,  as  before, 


m±<?>y= 


w'x1  tang2  ABD 


X  i* 


wV  tang2  ABD 
6 


(5) 


in  which  W  equal  to  the  weight  of  wall  and  earth  on  the  steps 
(assuming  a  plane  of  division  along  AB ),  to  be  determined  from 
the  cross-section  of  the  wall ;  small  w'  =  unit  weight  (1  cu.  ft.) 
of  the  material  supported  by  the  wall;  t  —  FB  =  thickness  of 

00 0  —  angle  of  repose  (0) 

wall,  and  angle  ABD  — - - - - - .  The  lever 


arm  of  the  external  pressure  =  \x  =  hB  ;  AB  =  x  —  height  of 
wall.  The  moment  of  stability  —  W{qt  ±  <7,/),  and  the  condition 

.  P 

of  frictional  stability  tang  vrt ,  =  rt^k  =  <  tang  0. 

ill.  Both  of  these  formulae  are  supposed  to  be  based  on 
erroneous  or  false  assumptions,  and  consequently  the  results 
are  not  considered  at  all  reliable.  They  are,  however,  approxi¬ 
mately  true  if  the  material  is  clean  sand.  As  a  simple  applica¬ 
tion  of  the  formulae  and  for  the  sake  of  comparison,  we  will 
assume  the  following  quantities  :  AB  =  x  —  20  ft.  height  of  the 
wall,  supporting  clay  in  a  fair  or  normal  condition,  surface  AC 


62 


A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


of  earth  horizontal.  The  wall  of  rectangular  horizontal  section 
and  vertical  section,  BC  the  natural  slope,  BD  the  plane  of 
rupture,  0  the  angle  of  repose  =  CBH  —  340  ;  then  ABD — 

9°  — —  28°,  and  tang  28°  —  0.53.  The  line  of  action  of  the 

weight  passing  through  the  centre  of  gravity  of  the  base,  or 
rather  centre  of  figure,  hence  ql  —  o;  weight  of  a  cubic  foot  of 


e  ad  p 


Fig.  5.—  Cross-section  of  Retaining-wall  Prism  of  Maximum  Pressure  Resultant. 


clay  =  w'  =  120  lbs.,  and  w  =  weight  of  a  cu.  ft.  of  masonry 
—  150  lbs.  W  —  wxt  —  1 50  X  20  X  t.  Substituting  these 
values  in  eq.  5,  par.  no,  -(-  q)t  =  -fcw'x3  tang2  ABD ,  we 
have  1 50  X  20  X  /  X  f/  =  }  X  1 20  X  8000  X  0.2809  ;  hence  t  = 
6.3  ft.  —  thickness  of  wall.  Now  substituting  in  eq.  2  (Rankine’s), 

_  w'x'1  cos  6  p 

+  ?iV  = - 2 - x  J  x  (k*  cos  6  ~  (?  +  h)*  sin  0, 


recollecting  that 

px  _  cos  6  —  V cos2  8  —  cos2  0'  1  —  sin  0' 
p  cos  8  y' cos2  8  —  cos2  0'  1  -j-  sin  0'’ 


RE  TA INING-  WA  LLS.  63 

since  <9  =  0;  cos  6 
340  =  0.56; 

Substituting, 

150  X  20  X  t  X  f* 

The  formulae  reducing  to  the  same  value  under  these  conditions, 
and  as  the  angle  of  repose  assumed  is  about  that  for  dry  sand, 
the  formulae  give  fairly  good  results.  The  least  thickness  in 
practice  would  be  of  20  —  6.6  ft.,  and  more  generally  would 
be  f  of  20  —  8.0  ft.  For  wet  clay  or  quicksand  the  formulae 
give  about  9.0  ft.,  but  in  practice  it  should  not  be  less  than  15 
to  18  ft.  These  formulae  will  generally  give  a  less  thickness 
than  would  be  good  practice.  It  will  be  observed  that  the 
axis  about  which  moments  have  been  taken  is  at  a  point  ■§■  of 
the  thickness  of  the  wall  from  the  outer  edge.  If  it  had  been 
taken  at  the  outer  edge  the  moment  to  resist  overturning 
would  have  been  a  little  greater,  and  consequently  the  result¬ 
ing  thickness  would  have  been  a  little  less  than  those  obtained. 
In  applying  Rankine’s  formula  for  fluid  pressure  the  only 
changes  necessary  would  be  in  the  value  w'  from  120  lbs.  to 
'■ 62 £  lbs.  per  cubic  foot,  and  in  making  <p'  and  6  =  o;  then  sin 

p 

4>'  —  sin  6  —  o,  and  —  would  become  unity.  The  substitu- 

P 

tion  would  give 

62.5  X  400 

1 50  X  20  X  t  x  1 1  — - — - X  |  X  20. 

i  =  8.6  (should  be  10  ft.)  thickness  of  wall  for  water  pressure. 
Water  pressure  can  be  calculated  easily  in  any  case  by  multi¬ 
plying  the  area  of  the  immersed  surface  by  the  depth  to  which 
its  centre  of  gravity  is  immersed  and  by  the  weight  of  a  cubic 
foot  of  water  (=  62^  lbs).  The  point  of  application  of  resultant 
pressure  is  one  third  of  the  height  of  the  wall  from  the  bottom. 
The  direction  of  the  pressure  is  always  perpendicular  to  the 
surface  pressed,  and  this  is  true  whether  the  surface  is  vertical, 
inclined,  or  horizontal. 


=  1.  And  sin  6  =  0,  also  the  sin  0'  =  sin 

A  1  “  -56 


P  1  +-56 
120  X  400  X  0.28 


—  0.28. 


X  i  X  20,  or  t  —  6.3  ft. 


6 4  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


1 12.  In  the  case  of  surcharged  retaining-walls,  from  the  tops 
of  which  the  slopes  of  the  embankments  rise  at  the  angles  of 
repose,  such  as  terraces  supported  by  walls  near  the  bottom, 
or  in  masonry  walls  surmounted  by  embankments,  as  in  the 
case  of  forts,  even  theory  seems  to  be  silent  on  this  subject. 
The  only  practical  rule  is  to  be  sure  to  make  them  thick 
enough.  In  placing  the  earth  behind  retaining-walls  the  mate¬ 
rial  should  be  placed  in  thin  layers  and  well  rammed  for  at 
least  io  ft.  back  from  the  wall ;  it  then  may  be  dumped  in  the 
usual  way. 

113.  If  the  material  is  likely  to  become  like  quicksand  or 
soft  mud,  it  is  best  to  assume  it  as  a  fluid  having  the  weight  of 
the  solid  material  and  the  angle  of  repose  equal  to  zero,  as  in 
case  of  water  ;  this  will  make  the  thickness  from  i|-  to  2  times 
of  that  to  support  water. 

1 14.  Retaining-walls  are  of  three  kinds.  Where  the  wings 
are  inclined  to  the  face  of  the  wall,  they  are  simply  called  wing 
abutments  ;  where  they  extend  back  perpendicularly,  with  a 
hollow  space  between,  they  are  called  “  U  ”  abutments,  the 
hollow  to  be  filled  with  earth  ;  and  where  a  solid  stem  extends 
back,  they  are  called  “  T  ”  abutments.  There  is  no  advantage  in 
the  last,  and  requires  more  masonry,  as  a  rule.  The  T  abut¬ 
ments  should  be  left  hollow  for  about  2  ft.  from  the  top,  so  as 
to  allow  sand,  clay,  or  broken  stone  to  be  used  for  the  cross¬ 
ties  to  rest  on,  as  otherwise  a  jarring  disagreeable  motion  will 
follow  when  a  heavy  rolling  load  passes. 

Article  XII. 

ARCHES. 

115.  The  theory  of  arches  is  perhaps  as  little  understood 
as  in  ages  past.  Some  fall,  some  are  doubtless  in  a  precarious 
condition,  some  stand.  Mathematical  and  mechanical  theories, 
after  carrying  you  through  the  intricate  mazes  of  higher 
mathematics,  have  surely  led  to  no  satisfactory  or  practical 
results.  The  theory  of  graphics  is  more  pleasant  to  handle, 


ARCHES. 


65 


certainly  easier  to  grasp,  and  may  do  admirably  to  back  up 
guesses,  or  to  shift  responsibility  in  case  of  accident  or  failure. 
Mr.  Rankine,  after  going  through  a  most  able  and  wonderfully 
conceived  discussion  of  this  subject,  tells  you  to  make  your 
factor-of-safety  from  20  to  40,  and  closes  by  saying  :  “  The  best 
course  in  practice  is  to  assume  a  depth  for  the  key-stone  ac¬ 
cording  to  an  empirical  rule,  founded  on  dimensions  of  good 
existing  examples  of  bridges.”  We  might  inquire  here  how  the 
old  arch-builders  came  anywhere  near  safe  dimensions  Did 
they  understand  the  theory  of  the  arch  ?  or  did  they  arrive  by 
repeated  failures  or  disasters  to  what  at  any  rate  are  safe 
dimensions,  and  we  now  profit  by  their  experience  ?  We  must 
do  the  best  we  can,  but  be  sure  of  being  on  the  safe  side. 

Il6.  Having  fixed  upon  the  depth  of  the  key-stone,  the 
same  depth  is  maintained  down  to  the  springing  line,  in  small 
arches.  In  arches  of  long  span  the  depth  increases  gradually 
from  crown  to  springing  line,  so  as  to  maintain  the  unit  pressure 
the  same  throughout,  according  to  a  simple  and  well-known  law, 
that  at  any  bed-joint  the  resultant  pressure  will  be  the  hypo- 
thenuse  of  a  right-angled  triangle,  of  which  the  base  is  taken 
to  represent  the  horizontal  thrust  at  the  crown  and  the  altitude 
the  weight  on  that  portion  of  the  arch  ring  from  the  crown  to 
the  bed-joint  under  consideration  ;  and  by  proper  construction 
to  scale  on  the  drawing  itself,  the  direction  of  the  resultant  and 
centre  of  pressure  can  be  determined.  At  the  springing  the 
resultant  pressure  is  represented  in  direction,  magnitude,  and 
point  of  application,  which  three  elements  must  be  known,  by 
the  hypothenuse  of  a  right-angled  triangle,  the  base  being  the 
horizontal  thrust  at  the  crown,  the  vertical  being  the  weight  of 
the  half  arch  and  any  load  upon  it,  supposed  to  pass  through 
the  centre  of  gravity  of  the  mass.  Following  the  process 
above  mentioned  for  finding  the  centre  of  pressure  at  each  bed- 
joint  in  the  arch  ring,  the  line  passing  through  these  centres  of 
pressure  is  called  the  line  of  pressure,  which  for  safety  should 
be  confined  to  the  middle  third  of  the  thickness  of  the  arch 
ring.  Although  after  all  this  we  may  be  in  doubt  whether  the 
line  of  pressure  will  under  all  conditions  remain  where  we  put 


66 


A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


it,  yet  we  fortunately  do  know  by  experiment  and  observation 
the  manner  in  which  both  flat  and  pointed  arches  give  way ; 
and  though  we  may  have  blundered  in  determining  the 
thickness  of  the  arch  ring  and  the  proper  curve  to  which  it 
should  be  built,  the  above  knowledge  enables  us  to  correct  this 
error  by  what  is  known  as  the  backing.  Flat  arches  give  way 
by  breaking  into  four  parts, — opening  at  the  crown  of  the  arch 
on  the  underside  or  the  intrados,  and  opening  on  either  side  at 
a  joint  not  definitely  known,  on  the  top  or  extrados,  but  never 
above  that  point  which  makes  an  angle  of  45  with  the  horizon, 
the  two  uppei  parts  falling  inwards  and  pressing  the  two  lower 
parts  outwards.  This  last  can  be  prevented  by  carrying  up  the 
masonry  of  the  abutments  above  the  point  mentioned,  and  this 
in  turn  preventing  the  upper  parts  from  falling. 

1 17.  In  pointed  arches  the  condition  is  just  reversed,  the 
two  lower  parts  falling  inwards  and  tending  to  lift  the  upper 
parts.  This  can  be  prevented  by  weight  of  sufficient  magnitude 
on  top ;  so,  notwithstanding  the  ignorance  on  the  subject  of 
arches,  by  due  precaution  we  can  feel  reasonably  safe  as  re¬ 
gards  the  stability  of  any  given  arch. 

118.  The  almost  universal  rule  is  to  build  the  ring  of  the 
arch  of  the  very  best  kind  of  ashlar  masonry,  cut  so  that  the  ring 
stones  may  bear  against  each  other  with  the  thinnest  possible 
joints,  which  can  be  filled  with  grout  or  at  least  very  thin  mori 
tar.  The  backing,  the  abutments,  and  the  spandrels,  or  the 
wall  resting  on  the  arch  ring,  can  be  built  of  a  less  costly  class 
of  masonry. 

119.  Applying  the  principles  explained  in  discussing  retain- 
ing-walls,  the  direction  of  the  pressure  is  towards  the  back  of 
the  wall  rather  than  the  face  ;  hence  the  back  of  an  abutment 
carrying  an  arch  should  be  built  of  ashlar,  or  at  least  a  good 
class  of  masonry.  The  exposed  face  need  not  be  so  good,  but 
for  appearance’  sake  it  is  generally  ashlar  ;  but  in  the  case  of 
arches  the  abutment  is  generally  supported  on  the  back  by  an 
embankment,  and  the  same  care  is  not  necessary  in  building 
the  back  or  unexposed  face  of  the  masonry. 

120.  In  building  an  arch  ring  it  should  be  built  from  both 


ARCHES. 


67 


abutments  at  the  same  time  upwards  towards  the  crown,  con¬ 
sequently  the  arch  ring  has  to  be  supported  until  the  arch  is 
completed,  that  is,  when  the  key-stone  is  put  in.  The  frame¬ 
work,  generally  of  timber,  which  supports  the  arch  ring  is  called 
a  “  Centre  this  centre  generally  remains  in  place  until  the 
cement  has  had  time  to  set,  and  is  then  removed.  The  frame 
generally  rests  on  wedges  ;  these  being  driven  out  gradually,  the 
centre  falls  from  the  arch  without  shock  or  jar. 

121.  Arches  are  generally  built  over  streams,  roads,  open¬ 
ings  such  as  doors  and  windows  in  houses  ;  and  sometimes  where 
large,  heavy  masses  of  masonry,  such  as  unusually  large  piers  or 
abutments,  are  to  be  built,  requiring  large  quantities  of  masonry, 
the  amount  of  masonry  . can  be  materially  diminished  by  the 
use  of  arches,  without  injuring  the  stability  of  the  structure. 
Where  the  stone  of  which  it  is  built  is  strong  and  hard  enough 
to  bear  the  superincumbent  weight  on  a  considerably  reduced 
area  of  bearing  surface,  and  where  also  the  foundation  bed  can 
bear  the  increased  unit  pressure,  this  can  be  reduced  by  the 
use  of  inverted  arches  under  the  arch  proper.  Stone  arches  of 
great  span  are  not  now  built  to  the  same  extent  as  they  were 
formerly,  iron  and  steel  having  been  substituted  to  a  very  great 
extent,  and  generally  as  horizontal  trusses. 

122.  With  no  exact  mathematical  formulae  to  guide  us  in 
the  construction  of  arches,  we  are  mainly  compelled  to  follow 
some  empirical  rule,  based  upon  the  dimensions  of  existing 
arches,  which  at  least  stand  though  we  do  not  know  the 
amount  or  direction  of  action  of  the  external  forces  or  loads, 
even  when  fixed  or  dead,  and  still  less  of  the  effect  of  heavy 
rolling  or  moving  loads.  We  can,  however,  approximate  to 
these  ;  and  with  the  knowledge  that  the  arch  ring  must  give  way 
either  by  crushing  the  voussoirs  or  arch  stones,  or  by  the  slid¬ 
ing  of  one  stone  on  another,  or  by  the  arch  ring  rotating  or 
revolving  inwards  or  outwards  around  the  inner  or  outer  edges 
of  some  of  the  stone  as  an  axis,  we  can  arrive  at  a  safe  thick¬ 
ness  of  the  arch  ring  and  the  proper  form  of  the  arch  ;  and 
in  general  we  boldly  assume  the  form  of  the  arch  ring  and  the 


68 


A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


thickness  of  it,  and  by  a  tentative  process  determine  whether  it 
will  be  stable  under  the  conditions  assumed. 

We  know  the  resistance  to  crushing  of  the  stone,— this  must 
not  be  exceeded  by  the  greatest  pressure  to  which  it  is  subjected 
after  allowing  a  large  factor  for  safety, — and  that  it  should  be 
distributed  as  uniformly  over  the  bearing  surface  of  the  stone  as 
possible,  and  for  this  the  centre  of  pressure  should  be  as  near  the 
centre  of  the  bearing  surface  as  possible.  To  resist  overturning 
around  the  edge  of  anyjointthe  centre  of  pressure  must  not  be 
above  or  below  the  arch  ring  at  any  point,  but  must  be  on  the 
surface  of  the  stone,  and  as  near  the  centre  of  figure  as  possible — 
at  any  rate  within  the  middle  third  of  the  arch  ring.  To  prevent 
sliding,  the  resultant  pressure  at  any  bed-joint  must  not  make 
an  angle  greater  than  the  angle  of  repose  with  the  normal  to 
the  bed-joint  at  that  point.  The  backing  should  be  built  up 
above  that  joint  which  makes  an  angle  of  45  with  the  vertical  : 
this  backing  is  generally  carried  up  to  the  crown,  gradually 
thinning  as  it  approaches  the  top.  All  of  these  conditions  be¬ 
ing  fulfilled,  the  engineer  may  feel  reasonably  safe  as  to  the 
stability  of  an  arch  ;  if,  however,  the  graphical  solution  of  the 
problem  fails  in  any  of  the  above  respects,  the  arch  ring  must 
be  made  thicker  or  the  form  of  the  curve  changed,  or  both. 

123.  It  will  be  best  to  define  the  terms  that  will  be  used. 
The  arch,  taken  as  a  whole,  consists,  1st,  of  the  abutments 
from  which  the  arch  springs  ;  the  top  of  the  abutment  on  the 
inner  edge  is  called  the  springing  line  ;  the  truncated  wedge- 
shaped  stones  resting  on  the  abutment  are  called  Skew-backs  ; 
2d,  of  the  arch  ring  itself,  composed  of  wedge-shaped  stones, 
called  voussoirs  or  ring-stones,  of  varying  sizes,  but  for  the  same 
arch  the  breadth  should  be  as  uniform  as  practicable;  lengths 
should  vary  in  order  to  get  bond;  the  under  side  of  the  stones 
are  cut  to  the  curve  of  the  arch.  The  under  or  cylindrical  sur¬ 
face  of  the  arch  is  called  the  Intrados  or  Soffit.  The  upper 
surface  or  back  of  the  stones  (generally  left  rough,  conforming 
roughly  to  the  curve  of  the  ring  of  the  arch)  is  the  Extrados  or 
back.  The  thickness  of  the  ring  is  determined  by  the  depth 
of  the  surfaces  in  actual  contact  included  between  two  parallel 


ARCHES. 


69 


curves,  the  intrados  and  extrados  proper.  The  exposed  under 
surface  is  generally  dressed  smooth ;  the  joints  between  the 
voussoirs  are  always  cut  true.  The  bed-joints  or  surfaces  of 
contact  of  the  stones  radiate  from  the  centre  or  centres  of  the 
curves  of  the  arch  ring.  The  Key-stone  is  at  the  top  of  the 
arch  or  crown  ;  it  is  the  last  stone  put  in,  and  the  arch  is  not 
self-supporting  until  it  is  in  place.  The  face  of  the  arch  is  its 
end  or  head,  the  axis  is  the  centre  line,  perpendicular  to 
the  head  of  the  arch  in  square  arches,  and  oblique  in  Skew 
arches.  A  ring-course  is  a  portion  of  the  arch  ring  included 
between  two  vertical  planes  perpendicular  to  the  axis  or  par- 
rallel  to  the  head  of  the  arch,  and  at  any  distance  from  each 
other, — say  a  foot  or  two.  A  String-course  is  that  portion  of  the 
arch  ring  included  between  two  inclined  planes  extended  from 
end  to  end  of  the  arch  and  intersecting  in  the  axis  of  the  arch, 
these  planes  containing  the  contiguous  joints  between  the 
stones.  The  centres  of  pressure  are  the  points  in  which  the  re¬ 
sultant  pressures  pierce  the  joints  between  the  stones,  and  the 
line  of  pressure  is  the  curved  line  passing  through  them.  In 
large  arches,  especially  when  flat,  the  thickness  of  the  ring- 
stone  increases  from  the  crown  to  the  springing  as  the  secant 
of  the  angle  of  inclination  of  the  curve  at  any  point,  and  at  the 
springing  maybe  times  that  at  the  crown  ;  in  circular  arches, 
if  small,  no  increase  is  made  generally. 

124.  A  Full  centre  arch  is  a  semicircumference  in  cross-sec¬ 
tion,  and  is  rarely  used  except  for  comparatively  small  arches, 
.as  the  rise  would  be  too  great,  if  for  no  other  reason.  The 
Segmental  arch  is  flat,  and  generally  a  segment  of  one  circle,  with 
a  long  radius,  and  is  called  a  one-centre  arch  ;  sometimes  it  is 
composed  of  segments  of  three  circles,  the  upper  part  of  a  long 
radius,  and  the  portions  near  the  springing,  called  the  Haunches, 
having  short  and  equal  radii :  this  approaches  the  elliptical  form, 
and  is  generally  called  the  elliptical  arch  ;  the  true  ellipse  may 
be  used.  The  span  of  the  arch  is  the  horizontal  distance  from 
the  springing  line  to  springing  line.  The  Rise  of  the  arch  is 
the  vertical  distance  from  the  springing  line  to  the  soffit  at  the 
crown. 


A  PRACTICAL  'TREATISE  ON  FOUNDATIONS. 


7  o 


125.  The  Spandrel  Wall  or  parapet  wall  is  not  an  essential 
part  of  the  arch,  but  is  built  to  give  a  finish  to  the  ends,  and  at 
the  same  time  if  an  embankment  is  built  over  the  arch  it  serves 
as  a  retaining-wall  for  the  foot  of  the  slope  of  the  embankment. 
It  is  a  wall  3  or  4  feet  high,  built  over  the  ends  of  the  arch  ring 
and  in  the  plane  of  the  arch,  and  is  finished  with  a  coping  ;  it, 
with  the  wings,  prevents  the  earth  from  rolling  over  or  around 
the  ends  of  the  arch  ;  it  supports  a  surcharge  embankment  and 
should  be  thicker  than  that  of  a  retaining-wall  of  that  height. 
Sometimes  intermediate  walls  are  built  parallel  to  the  head 
walls.  The  backing  between  the  head  walls  is  really  a  part  of 
the  arch  proper. 

126.  To  determine  the  length  of  an  arch,  from  end  to  end 
supporting  an  embankment  above,  the  slope  of  the  embankment 
being  l|  to  I,  it  is  merely  necessary  to  deduct  from  the  width 
of  the  embankment  at  the  bottom  three  times  the  height  from 
the  ground  line  to  the  top  of  the  spandrel  wall ;  the  arch  should 
be  a  little  longer  than  this  difference.  The  wing  walls  then 
extend  to  the  foot  of  the  slope. 

127.  The  masonry  of  the  abutment  is  generally  faced  with 
ashlar,  block-in-course,  or  coursed  rubble  masonry  ;  theoreti¬ 
cally,  the  back  should  be  equally  good  or  better  than  the  face, 
but  is  generally  of  a  rougher  finish.  Owing  to  the  fact  that  the 
thrust  of  the  arch  tends  to  overturn  the  wall  in  one  direction 
and  the  embankment  in  the  other,  the  direction  of  the  result¬ 
ant  may  be  inclined  either  way,  or  may  be  vertical  according 
to  their  relative  magnitudes.  The  embankment  should  be  built 
on  both  sides  of  the  arch  at  the  same  time,  and  should  be 
rammed  in  layers  around  and  over  the  arch  for  at  least  10  feet 
in  thickness,  after  which  the  earth  may  be  dumped  in  the 
usual  way.  It  is  customary  to  pile  up  behind  the  abutments 
the  shivers  of  rock  and  dbbris  accumulating  around  the  work. 
This  facilitates  the  drainage,  and  at  the  same  time  strengthens 
the  wall  until  the  embankment  is  built. 

128.  In  small  arches,  unless  the  bed  of  the  stream  is  rocky, 
it  is  best  to  pave  the  bottom  between  the  walls  with  stone 
from  6  to  12  inches  thick,  and  also  to  build  apron  walls  under 


ARCHES. 


71 


the  ends  deeper  than  the  foundation  bed  of  the  abutments,  to 
avoid  any  danger  of  scouring.  The  spans  for  such  arches  gen¬ 
erally  vary  from  5  to  20  feet,  and  are  generally  full-centre 
arches.  For  longer  spans  it  will  generally  be  economical  to 
use  the  flat  or  segment  arch,  and  avoid  too  great  a  rise  of  the 
arch. 

129.  The  arch  ring-stones  are  all  dressed  true  on  the  joints 
and  soffit,  and  are  of  best  kind  of  ashlar.  The  width  of  the  ring- 
stones  is  seldom  less  than  one  foot  or  more  than  three  feet,  and 
the  thickness  or  depth  from  one  to  five  feet.  The  upper  sur¬ 
face  is,  in  general,  covered  with  a  layer  of  cement  or  asphalt, 
or  some  waterproof  substance  to  drain  the  water  to  the  proper 
drains,  and  prevent  the  dripping  that  would  otherwise  pass 
through  the  joints  of  the  ring.  The  stones  of  the  arch  ring- 
break  joints  in  the  direction  of  the  length  of  the  arch.  Arch 
stones  are  often  cut  and  marked  so  as  to  fit  in  a  certain  posi¬ 
tion,  as  shown  on  the  development  of  the  arch  on  paper;  this 
is  convenient,  and  saves  time  and  trouble.  The  development 
of  a  square  arch  would  be  a  rectangle,  one  side  of  which  is  the 
length  of  the  arch,  and  the  other  side  is  the  length  of  the  arch 
ring  itself,  upon  which  the  string-courses  can  be  laid  down  to 
scale  in  their  true  positions,  and  arranged  so  as  to  secure  the 
proper  bond,  and  the  arch  ring  should  be  built  to  correspond. 
The  ring-coursed  stone  on  the  ends  of  the  arch  are  cut  on  top 
to  vertical  and  horizontal  surfaces  in  order  to  let  the  spandrel 
wall  rest  true  on  the  ring,  and  also  to  bond  with  it,  also  for 
appearances. 

130.  The  masonry  of  the  spandrel  may  be  ashlar  or  block- 
in-course  masonry,  backed  with  rubble.  The  wings  may  be  the 
same  kind  of  masonry,  and  proportioned  as  in  retaining-walls. 
The  backing  is  generally  of  heavy  rubble  or  may  be  made  of 
concrete,  rounding  off  towards  the  crown  of  the  arch,  and  cov¬ 
ered  over  with  a  layer  of  cement. 


72 


A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


Art.  XIII. 

SKEW  ARCHES. 

131.  The  skew  arch  is  built  of  the  same  parts  and  of  the 
same  kind  of  masonry  in  the  corresponding  parts,  but,  owing 
to  the  inclination  of  the  axis  of  the  arch  to  the  plane  of  the 
face,  the  string-course  joints  are  curved,  and  each  joint  is  of  a 
different  kind  of  curve — that  is,  a  series  of  irregular  spirals 
drawn  perpendicular  to  the  lines  of  pressure  in  different  sec¬ 
tions  of  the  length  of  the  arch,  taken  at  convenient  intervals; 
and,  although  the  general  conditions  of  stability  are  the  same  as 
in  a  square  arch  of  the  same  span,  the  above  directions  of 
the  bed-joints  require  every  stone  in  the  arch  to  be  of  a 
different  size  and  shape,  with  all  surfaces  curved.  Here  the 
knowledge  of  descriptive  geometry  and  stereotomy  are  re¬ 
quired  to  determine  the  shape  of  the  stone,  and  to  construct 
the  templets  to  guide  the  stone-cutters.  This  is  troublesome 
and  laborious,  and  requires  great  accuracy  and  care,  as  each 
stone  will  fit  in  but  one  position  in  the  arch  ring ;  the  cutting 
is  expensive,  the  building  is  troublesome  and  slow,  the  whole 
structure  is  costly;  hence  engineers  avoid  as  much  as  possible 
the  use  of  this  arch,  and  have  so  modified  its  construction  as 
to  avoid  these  difficulties. 

132.  Only  a  general  outline  of  the  first  method  will  be 
given,  the  method  of  constructing  the  development  of  the 
soffit  will  be  found  in  Rankine’s  Civil  Engineering,  pages  450 
and  451. 

133.  The  first  thing  is  to  draw  to  a  large  scale  the  develop¬ 
ment  of  the  soffit  of  the  arch.  A  ring  or  wheel  when  revolved 
once  develops  a  straight  line,  equal  in  length  to  the  circum¬ 
ference  of  the  wheel;  a  right  cylinder,  or  take  a  semi-cylinder, 
when  revolved  develops  a  rectangle,  the  length  of  which  is  the 
length  of  the  arch,  and  the  breadth  is  the  length  of  the  semi¬ 
circumference  on  the  soffit  of  an  arch  ;  and  an  oblique  cylin¬ 
der,  or  the  skew  arch,  when  revolved  will  develop  a  figure  ap- 


ARCHES. 


73 


proaching  the  rectangular  in  shape,  with  two  straight  parallel 
-sides  equal  in  length  to  the  length  of  the  arch,  the  other 
sides  parallel,  but  curved,  and  equal  to  the  length  of  the 
soffit. 

134.  A  full  discussion  of  arches,  development  of  the  soffit, 
lines  of  pressure,  ring  and  string  course  joints,  thrust  or  pres¬ 
sure  at  the  crown  and  at  other  points,  etc.,  will  be  found  in 
another  volume. 


Article  XIV. 


DEPTH  OF  KEYSTONE. 


135.  There  are  many  methods  and  theories  on  this 
subject,  but  as  none  of  them  lead  to  better  or  more  certain 
or  more  reliable  results,  the  reader  is  referred,  for  full  dis¬ 
cussions,  to  such  authors  as  Rankine,  Weisbach,  Moseley.  In 
practice  empirical  formulae  are  used.  Trautwine  gives  the 
following  practical  formula  for  determining  depth  of  arch 
ring  at  the  crown  :  Depth  of  key-stone  in  feet  equal  to 


V  radius  -j-  £  span 


-j-  0.2  ft.  for  first-class  cut-stone  work. 


Increase  this  result  by  part  for  second-class  work,  and  for 
brick  or  rubble  \  part.  The  depth  of  the  arch  ring  should  in¬ 
crease,  theoretically,  from  the  crown  to  the  springing,  this 
increase  at  the  springing  being  from  one-fourth  to  one-half  the 
depth  at  the  crown,  but  is  never  necessary  for  small  arches. 
Rankine’s  formula  is:  Depth  of  key-stone  in  feet  equal  to 
Vo.  1 2  X  radius  at  crown  ;  and  for  an  arch  of  a  series  the  depth 
of  key-stone  in  feet  equal  toVo.iy  X  radius  at  crown.  For 
tunnels,  which  generally  have  elliptical  cross-sections,  depth  of 


key-stone  in  feet  equal  to  Vo A2r,  in  which  r  ~ 


c? 

!>' 


in  which  a 


is  the  rise  of  the  arch  from  two  thirds  to  three  fourths  of  the 
transverse  diameter  and  b  is  the  semi-conjugate  diameter. 


74 


A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


In  soft  and  slippery  materials  the  thickness  should  be  doubled. 
Assuming  a  radius  at  the  crown  of  160  ft.  and  span  147.6  ft.,. 
Trautwine’s  formula  gives  depth  of  key-stone  equal  to  4.0  ft. 
and  Rankine’s  4.4  ft.;  actual  thickness  4.9  ft.  This  was  a 
segmental  arch.  Again,  by  Rankine,  an  elliptical  arch  30  ft. 
span,  and  rise  7%  ft.,  calculated  thickness  1.9  and  actual  thick¬ 
ness  2  feet;  a  90-ft.  span  segmental  arch,  rise  30  ft.,  calculated 
thickness  2.88,  actual  thickness  of  key-stone  30  ft.  About  the 
largest  arch  built  is  the  Cabin  John  aqueduct,  Washington,  D.  C. 
Span  220  ft.,  rise  57.25  ft.,  radius  at  crown  134.25  ft.,  thickness 
of  arch  ring  at  the  crown  4.16  ft.,  and  at  the  springing  6.0  ft.  ; 
segmental  in  form.  For  depth  of  this  arch  at  crown  Traut¬ 
wine’s  formula  gives  4.1  ft.,  and  Rankine’s  4.0  ft.  Therefore 
we  may  safely  conclude  that  either  formula  gives  safe  results 
in  practice. 

136.  To  what  extent  and  in  what  manner  a  heavy  rolling 
load  affects  the  line  of  pressure  or  the  stability  of  an  arch  is 
not  known,  but  in  very  large  and  heavy  arches,  or  where  there 
is  a  great  depth  of  earth  over  the  top,  it  probably  causes  no 
great  change.  Several  feet  of  earth  or  ballast  should  be  placed 
over  an  arch  to  prevent  the  effects  of  shocks  from  a  rapidly 
moving  train.  Arches  are  built  of  masonry,  iron,  or  wood  ;  the 
same  general  principles  are  applicable. 

The  above  conditions  are  necessary  to  prevent  overturning 
around  the  edge  of  any  joint.  To  prevent  sliding  at  any  joint, 
the  direction  of  the  resultant  pressure  must  not  make  with 
normal  to  that  joint  a  greater  angle  than  the  angle  of  repose 
or  of  friction  of  stone  on  stone  ;  this  is  not  likely  to  take  place 
unless  the  abutment  settles. 

137.  In  the  above  considerations  no  account  is  taken  of  the 
tenacity  of  the  mortar  or  its  adherence  to  the  stone,  which 
would  add  materially  to  the  strength  of  the  arch. 

138.  The  conditions  of  stability  of  the  abutment  is  the  same 
as  that  of  a  retaining-wall  acted  upon  by  a  resultant  pressure 
equal  in  magnitude,  direction  and  point  of  application  of  the 
resultant  pressure  of  the  arch  at  the  springing.  By  building 
the  abutments  in  courses  radiating  from  the  centre  of  the  arch. 


BRICK. 


7$ 


the  line  of  pressure  in  the  abutment  would  be  a  continuation 
of  the  line  of  pressure  in  the  arch  ring,  this  should  be  confined 
in  the  middle  third  of  the  abutment,  and  when  the  courses 
are  horizontal  is  an  approximate  continuation  of  that  line. 
The  flatter  the  arch  the  greater  will  be  the  tendency  to  over¬ 
turn  the  abutments. 


Article  XV. 

BRICK. 

139.  Brick  Walls  and  Piers  — Stone  is  always  preferred  for 
large  piers  and  abutments,  but  in  many  parts  of  the  country, 
especially  in  many  Southern  States,  brick  has  to  be  relied  upon 
for  almost  all  purposes  ;  and  in  all  parts  of  the  country  brick  is 
very  largely  used  for  private  dwellings  as  well  as  for  many 
public  buildings.  Brick  can  be  called  an  artificial  stone.  The 
principal  ingredients  in  brick  are  clay,  sand,  protoxide  of  iron. 
Other  substances  that  may  enter  into  ordinary  clay  either  do 
no  good  or  are  absolutely  harmful,  carbonate  of  lime  in  any 
large  proportions  rendering  the  clay  absolutely  unfit  for  making 
brick.  Sand  should  not  exist  in  any  excessive  quantity.  Protox¬ 
ide  of  iron  causes  the  red  color  in  brick  after  burning,  and  also 
increases  the  strength  and  hardness. 

140.  In  making  brick  the  clay  is  reduced  to  a  state  of  rather 
stiff  mud  with  water,  then  placed  in  what  is  called  a  “  pug-mill,” 
which  consists  essentially  of  a  vertical  cylinder,  in  the  centre 
of  which  is  a  vertical  shaft  with  radiating  arms,  so  shaped  and 
fixed  that  on  turning  the  shaft,  generally  by  a  horse  hitched  to 
the  end  of  a  lever,  the  clay  is  thoroughly  kneaded,  and  at  the 
same  time  forced  downwards  to  the  bottom  of  the  mill,  where 
it  is  passed  out  of  an  aperture  on  to  a  platform,  where  it  is  then 
pressed  into  a  mould  of  suitable  size  and  shape.  It  is  then 
placed  on  an  open,  well-prepared  yard,  where  it  is  sun-dried  for 
a  short  time.  When  properly  dried  (it  constitutes  something 
similar  to  the  “  adobe,”  which  was  formerly  used  for  construct- 


76  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 

ing  houses)  the  bricks  are  built  in  a  large  mass  or  kiln  of  a 
certain  established  width  and  height,  and  of  a  length  depending 
upon  the  number  of  bricks,  varying  from  100,000  to  300,000; 
eyes  or  flues  are  left  at  the  bottom  as  receptacles  for  fuel — in 
ordinary  cases  wood.  The  bricks  are  laid  rather  open,  so  as  to 
create  a  draft  and  allow'  the  heat  to  pass  in  and  around  them. 
When  ready  the  fire  is  started  slowly  at  first  and  increased 
to  an  intense  heat,  and  after  burning  for  a  period  determined 
partly  by  the  fuel  used,  but  mainly  by  experience,  the  fires  are 
allowed  to  die  out  gradually. 

141.  On  opening  a  brick  kiln  after  burning  the  quality  of 
the  brick  may  be  divided  into  four  classes,  extreme  outside 
brick,  on  sides  and  top  being  burnt  so  little  that  they  may  be 
thrown  away  as  worthless ;  then  a  layer  inside  of  the  above,  of 
more  or  less  thickness,  in  w'hich  the  brick  are  under  burnt  and 
soft ;  these  are  called  pale  or  salmon  brick,  unfit  for  foundations 
or  face  work,  but  are  used  for  filling  in  between  good  bricks  in 
walls.  In  the  centre  of  the  mass  forming  the  kiln  a  class  of 
brick  is  found  well  burnt,  hard,  well  shaped,  and  of  good  red 
color  ;  this  class  of  brick  is  good  for  any  purpose.  The  lower 
part  of  the  kiln  just  above  the  eyes  are  over-burnt,  very  hard, 
very  brittle,  and  generally  distorted,  cracked,  and  even  vitrified; 
these  are  not  suitable  for  structures  exposed  to  shocks.  The 
second  class  are  called  pale,  salmon,  or  under-burnt  brick,  very 
soft  and  porous.  The  third,  body  or  red  brick,  hard  and 
strong  and  used  in  face  of  wall. 

142.  Bricks  are  used  for  houses  to  a  much  larger  extent 
than  any  other  materials  except  wood.  The  walls  of  houses 
are  generally  carried  up  plumb  or  vertical,  the  dimensions  at  the 
bottom  being  determined  by  the  nature  and  height  of  the  walls, 
and  purposes  for  which  they  are  constructed, — warehouses, 
on  account  of  the  immense  weight  which  maybe  placed  on  the 
floors,  requiring  thicker  walls  than  dwelling-houses.  This  thick¬ 
ness  in  many  cities  is  fixed  bylaw,  which  doubtless  corresponds 
with  the  practice  very  closely  in  all  cities.  The  thickness  is  gen¬ 
erally  stated  as  follows:  8-inch  or  9-inch  walls  being  one  brick 
thick,  12  to  13  inches  being  I-J  brick  thick,  and  so  on.  Bricks 


BRICIC. 


77 


vary  a  little  in  size  in  different  parts  of  the  country,  but  gener¬ 
ally  in  the  following  limits :  between  8  and  9  inches  long,  4  to 
4^  inches  wide,  2\  to  3  inches  thick.  It  will  be  noticed  that 
the  proportions  of  the  several  dimensions  are  about  the  same  as 
for  ashlar  masonry.  In  ordinary  houses,  the  thickness  at  the  bot¬ 
tom  varies,  according  to  height,  between  1^  bricks  thick  (say 
13  inches)  to  4  bricks  thick  (32  inches),  and  decreasing  to  1 
brick  thick  at  the  top  for  houses  of  moderate  height,  and  ij 
brick  thick  for  very  high  houses.  This  decrease  is  not  made  as 
in  piers  by  a  regular  batter,  but  by  abrupt  changes  or  by 
offsets  from  story  to  story.  The  walls  of  warehouses  should 
be  thicker  than  the  above,  depending  upon  their  size  and  the 
purpose  for  which  they  are  used.  The  above  dimensions  refer 
to  the  body  of  the  wall  or  the  neat  work;  the  footing-courses 
about  double  the  above. 

143.  Brick-work  is  built  in  regular  courses  of  the  thickness 
of  the  brick,  well  bonded,  and  with  joints  of  not  over  \  of  an 
inch  thick,  or  it  is  better  controlled  by  saying  that  in  a  certain 
vertical  height  there  shall  not  be  more  than  so  many  courses 
of  brick.  If  the  brick  is  2f  inches  thick,  four  courses  should 
occupy  a  vertical  foot  on  the  face  of  the  wall. 

144.  There  are  two  kinds  of  bond,  the  English  and  the 
Flemish.  It  is  probably  immaterial  which  is  used  ;  but  in  the 
Flemish  bond,  where  stretchers  and  headers  alternate  in  each 
course,  a  more  certain,  uniform,  and  regular  bond  can  be  se¬ 
cured,  a  header  being  placed  immediately  over  a  stretcher 
below  ;  whereas  in  the  English  bond  the  headers  are  in  separate 
courses, — one,  two,  or  more  courses  of  stretchers,  then  one  of 
headers,  this  proportion  being  regulated  by  the  character  of 
the  work ;  a  factory  chimney,  for  instance,  requiring  a  larger 
proportion  of  stretchers  than  headers,  such  as  four  courses  of 
stretchers  to  one  of  headers.  This  kind  of  work  should  be 
rigidly  executed,  according  to  rules  established  both  by  theory 
and  practice.  More  latitude  can  be  given  in  the  case  of  ordi¬ 
nary  walls,  but  too  much  looseness  and  indifference  is  shown 
in  this  kind  of  work :  so  much  so,  that  it  is  often  impossible 
to  say  what  proportion  of  stsetchers  to  headers  is  allowed,  it 


78  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 

being  practically  left  to  the  masons  to  decide.  Mr.  Ran 
kine  says  one  course  of  headers  to  two  stretchers  gives 
equal  strength  longitudinally  and  transversely  in  the  English 
bond. 

145.  Often  in  brick  walls  stone  quoins  or  corner-stones  are 
put  in,  these  stones  being,  say,  10  or  12  inches  thick,  presumably 
to  strengthen  the  corners,  as  well  as  for  architectural  effect ; 
the  policy  in  either  case  is  at  the  best  doubtful. 

146.  A  great  difference  of  practice  in  building  walls  of 
houses  exists  in  regard  to  filling  the  joints,  and  the  common 
practice  is  to  make  good  beds  ;  smear  a  dab  of  mortar  on  the 
end  of  the  brick  before  placing  it  in  position,  leaving  even  the 
vertical  joints  of  the  bricks  unfilled,  and  no  attempt  being 
made  to  fill  the  back  joints  at  all ;  this  practice  certainly 
weakens  the  wall,  even  if  it  may  have  some  compensating  ad¬ 
vantages. 

147.  The  strength  of  ordinary  bricks,  such  as  the  hard,  red, 
well-burnt,  is  sufficient  to  resist  crushing  under  any  load  that 
is  likely  to  be  placed  on  it  in  the  walls  of  houses.  This  has 
been  amply  proved  by  experience  in  all  parts  of  the  country, 
and  these  bricks  must  have  been  made  of  clay  varying  largely 
in  their  composition,  and  both  by  hand  and  by  many  recently 
constructed  machines.  A  few  examples  will  suffice  to  establish 
the  truth  of  the  above.  Mr.  Rankine  gives  the  actual  existing 
pressure  at  the  base  of  a  chimney  450  feet  high,  20,000  lbs. 
per  square  foot,  or  140  lbs.  per  square  inch.  The  brick  shot- 
tower  in  Baltimore,  246  feet  high,  the  pressure  at  the  base  is 
about  13,000  lbs.  per  square  foot,  or  90  lbs.  per  square  inch, 
whereas  1100  lbs.  per  square  inch  is  considered  a  fair  ultimate 
strength  for  piers  or  walls  of  brick-work,  giving  a  factor-of- 
safety  of  from  8  to  12.  If  good  strong  cement  mortar  is  used, 
the  ultimate  strength  can  be  taken  at  from  1500  to  2000  lbs. 
per  square  inch;  the  factor-of-safety  will  be  at  least  from  n 
to  14. 

148.  Brick  piers  and  abutments  are  used  to  a  large  extent 
in  the  Southern  States,  on  account  of  the  difficulty  and  cost  of 
securing  stone  of  any  kind.  The  writer  built  a  bridge  across 


BRICK. 


79 


the  Tombigbee  River,  on  the  line  of  the  Mobile  and  Birming¬ 
ham  Railway  ;  the  piers  were  of  brick,  resting  on  concrete,  in 
the  cribs  of  pneumatic  caissons,  a  little  below  the  water  sur¬ 
face,  the  pressure  at  the  bottom  of  the  brick  piers  about 
7600  lbs.  per  square  foot  of  base ;  the  brick  were  almost  en¬ 
tirely  obtained  by  pulling  down  old  and  abandoned  ware¬ 
houses  in  Mobile,  Ala.  The  appearance  of  the  brick  indicated 
good  strong  brick,  and  the  time  that  they  had  stood,  with¬ 
out  signs  of  wear  or  disintegration,  indicated  durability;  in 
fact,  they  seemed  to  be  superior  to  the  new  brick  then  being 
made. 

149.  Good  brick  seem  to  be  as  durable  as  any  ordinary 
stone,  can  be  built  at  a  less  cost  per  cubic  yard  than  stone, 
and  resists  the  effect  of  intense  heat,  such  as  resulting  from 
fires  in  cities  ;  is  strong  enough  to  carry  any  load  likely  to 
occur ;  is  not  affected  by  acid  atmospheres  ;  and  now  that 
brick  can  be  obtained  of  different  colors  or  variegated  colors,  it 
would  seem  that  brick-work  can  satisfy  all  the  conditions  of 
strength,  durability,  and  architectural  effect  desired. 

150.  One  cause  of  the  apparent  distrust  in  brick-work  is 
that  manufacturers  of  brick  are  too  anxious  to  sell  a  poor 
quality  of  brick,  made  of  poor  material,  under  burnt  and  soft, 

'often  very  irregular  in  shape  and  size ;  angles  not  square, 
faces  not  parallel,  and  often  badly  warped  and  twisted.  These 
defects,  if  confined  to  the  backing  or  filling  of  the  wall,  would 
not  be  so  objectionable  ;  but  masons  are  too  apt  to  use  these 
on  the  faces  of  the  walls,  causing  ugly  joints,  irregular  courses, 
a  general  bad  and  rough  appearance  ;  and  in  the  filling  they 
will  use  brick  so  soft  that  they  are  unfit  for  any  purpose,  and 
would  soon  return  to  the  condition  of  mud  if  exposed.  Often 
to  get  a  sufficient  quantity  of  brick  you  are  compelled  to 
take  the  general  run  of  the  kiln.  These  things  cause  a  want 
of  confidence. 

151.  But  the  high  walls  of  houses  in  addition  to  resisting 
crushing,  have  to  resist  also  the  tendency  to  overturn,  either 
from  external  forces  from  within  or  without.  There  is  little 
or  no  danger  of  overturning  from  external  forces,  such  as 


So 


A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


winds,  as  the  floors,  partitions,  roofs,  etc.,  prevent  this;  over¬ 
weighted  floors  might  exert  this  tendency,  but  in  this  case  the 
floors  would  have  to  give  way  to  cause  any  dangerous  effects, 
but  at  the  same  time  there  would  be  an  outward  tendency 
also.  Both  of  these  effects,  whatever  be  their  relative  value, 
can  be  provided  against  by  some  simple  device  for  anchoring 
the  joists  to  the  walls,  which  would  at  the  same  time  give 
proper  play  for  expansion,  and  give  ventilation  to  the  ends  of 
the  joist.  One  form  of  this  is  an  iron  casting  with  a  rib  at  the 
bottom,  a  notch  being  made  in  the  joist  to  fit  over  it.  This 
anchoring  prevents  the  walls  from  overturning  outward  in 
warehouses  where  large  quantities  of  grain  and  other  like 
material  are  stored.  The  roof  also,  in  some  cases,  has  a  ten¬ 
dency  to  overturn  the  wall,  as  in  the  Gothic  roof-truss,  where 
no  tie-beam  is  used.  In  all  such  cases  the  thickness  of  the 
walls  must  be  increased,  or  it  must  be  stiffened  by  buttresses. 
A  standing  wall  of  brick  is  considered  the  best  and  surest  bar- 
rier  to  resist  the  spread  of  flames. 

152.  If  walls  of  brick,  such  as  house  walls,  give  way  by  slid¬ 
ing,  bulging,  or  overturning,  the  plane  or  line  of  breaking  will 
follow  the  mortar-joints,  as  the  resistance  to  the  tendencies 
depends  mainly  upon  the  adhesion  of  the  mortar  to  the  brick, 
or  the  tensile  strength  of  the  mortar  itself ;  therefore  for  the 
greatest  strength  the  mortar  used  should  be  at  least  as  strong 
as  the  brick  itself.  This  can  only  be  realized,  however,  by  the 
use  of  cement  mortar,  as  lime  mortar  will  never  be  as  strong  as 
good  brick,  and  as  too  often  used  is  not  much  better  than  so 
much  mud  ;  often  hardly  enough  lime  is  used  to  cover  the  grains 
of  sand,  and  it  is  not  unusual  to  see  the  mortar  eaten  out  from 
5  to  10  feet  above  the  ground,  apparently  caused  by  water 
absorbed  from  the  ground. 

153.  As  to  the  adhesion  of  the  mortar  to  the  brick,  it  is 
hard  to  determine  its  value,  as  it  depends  to  a  very  large  extent 
upon  the  character  of  the  brick  and  on  the  condition  of  its  sur¬ 
face  :  if  the  brick  is  dry  and  dusty,  the  adhesion  will  be  very 
small  ;  if  the  brick  is  porous,  clean,  and  wet,  it  will  be  of  consid¬ 
erable  value.  It  is  inexcusable  to  lay  brick  unless  they  are 


BRICK. 


81 


thoroughly  wet  or  even  saturated  with  water;  but  masons  will 
not  take  this  trouble  unless  compelled  to  do  so,  even  in  hot 
weather,  and  as  a  consequence  the  brick  separates  from  the 
mortar  with  a  perfectly  clean  surface.  In  the  contrary  cases  it 
is  often  difficult,  if  not  impossible,  to  remove  the  mortar  from 
the  faces  of  the  brick.  Always  wet  and  keep  the  brick  wet. 

154.  If  what  has  been  said  is  true,  brick  should  never  be 
used  below  ground  unless  good  cement  mortar  is  used,  and  it 
is  always  better  to  use  stone  even  then.  Dampness  can  be 
prevented  from  rising  in  the  walls  above  the  ground  by  using 
one  or  two  layers  of  slate  in  the  mortar-joints.  Lime  and 
cement  can  be  mixed,  using  one  barrel  of  lime  and  one  barrel 
of  cement,  or  even  two  of  lime  and  one  of  cement  would  be 
vastly  better  than  all  lime. 

155.  There  is  a  variety  of  hard  strong  bricks  called  com¬ 
pressed  bricks  :  these  are  generally  of  good  shape,  square  angles, 
true  and  parallel  surfaces,  made  in  different  parts  of  the  coun¬ 
try  ;  but  these,  owing  to  their  great  cost,  are  scarcely  used  ex¬ 
cept  for  facing  walls,  jams  and  lintels  of  doors  and  windows, 
cornices,  etc.  Walls  faced  in  the  ordinary  way  are  hardly  to  be 
recommended,  as  in  any  case  they  are  but  poorly  bonded  into 
the  back  of  the  wall  ;  this  arises  from  several  causes :  the  com¬ 
pressed  bricks  are  not  of  the  same  sizes  as  the  ordinary,  and  in 
addition  it  seems  to  be  the  practice  to  lay  the  facing  with  very 
thin  mortar-joints,  and  to  this  add  the  ordinary  carelessness  in 
building  the  back  walls.  The  monotonous  red  of  these  bricks 
and  the  unbroken  uniformity  in  color  does  not  always  add  to 
the  appearance  of  such  buildings. 

156.  A  good  safe  rule  for  the  thickness  of  the  walls  of 
houses  would  be  not  less  than  12  inches  at  top,  and  an  increase 
of  2  inches  for  each  12  feet  to  the  bottom  ;  this  for  a  wall  50 
feet  high  would  be  20  inches  at  the  ground  line.  It  should 
seldom  be  thinner  than  this,  and  for  warehouses  and  depots 
it  should  be  from  1^  to  2  times  the  above,  according  to  cir¬ 
cumstances. 

157.  Brick  is  also  largely  used  in  sewers,  which  are  gener¬ 
ally  circular  or  oval  in  cross-section. 


82 


A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


158.  Brick  pavements  for  streets  have  been  used  to  a  large 
extent  in  some  localities  and  seem  to  give  satisfaction,  are 
claimed  to  be  durable,  easily  cleaned,  comparatively  noiseless, 
and  favoring  a  good  foothold  for  horses.  Bricks  for  this  pur¬ 
pose  should  be  hard  and  sound,  and  of  the  best  quality,  as  they 
have  to  stand  wear  from  both  shocks  and  friction,  to  which 
ordinary  structures  are  not  exposed.  For  this  purpose  there 
is  not  perhaps  sufficient  experience  or  data  to  make  a  com¬ 
parison  with  other  paving  materials. 


Art.  XVI. 

BRICK  ARCHES. 

159.  BRICKS  are  used  very  largely  in  building  arches,  es¬ 
pecially  over  openings  in  ordinary  houses,  such  as  doors  and 
windows,  and  at  the  bottom  of  walls  to  keep  them  from  being 
pressed  inwards,  and  at  the  same  time  they  serve  to  distribute 
the  pressure  over  the  space  between  the  walls  ;  in  this  case  they 
afe  called  inverted  arches. 

160.  Ordinarily  arch  rings  of  brick  consist  of  one,  two,  or 
more  rings  of  brick,  laid  as  stretchers,  these  rings  being  only 
held  together  by  the  adhesion  of  the  mortar  between  them  to 
the  brick,  and  by  the  tenacity  of  the  mortar.  As  seen  above, 
the  line  of  pressure  in  an  arch  is  not  always  a  symmetrical  or 
regular  curve,  and  consequently  the  entire  pressure,  or  at  any 
rate  a  large  part  of  it,  will  be  concentrated  on  one  of  the  rings 
of  brick,  which  might  result  in  crushing  the  brick  or  in  separat¬ 
ing  the  rings. 

161.  There  are  only  two  methods  by  which  this  difficulty 
can  be  overcome  :  1st,  by  having  wedge-shaped  bricks  made  es¬ 
pecially  for  the  purpose,  which  can  either  be  equal  to  the  thick¬ 
ness  of  the  arch  ring,  or  can  be  laid  as  header  and  stretcher, 
thereby  distributing  the  pressure  ;  2d,  the  arch  can  be  so  built, 
by  regulating  the  thickness  of  the  joints,  that  at  intervals  the 
radiating  joints  of  the  several  rings  shall  be  in  the  same  plane, 


BRICK  ARCHES.  83 

so  that  headers  may  be  introduced,  resulting  in  a  distribution 
of  the  pressure. 

162.  Owing  to  the  fact  that  arches,  properly  speaking,  are 
built  according  to  the  curve  of  a  circle  or  an  ellipse  or  a  com¬ 
bination  of  these,  the  outside  rings  will  be  a  little  longer  than 
the  inner  rings,  and  as  a  consequence  with  ordinary  bricks  the 
joints  will  have  to  be  a  little  thicker  on  the  outside  rings.  This 
is  regulated  by  laying  the  inner  rings  with  a  very  little  space 
between  the  bricks,  and  gradually  increasing  in  thickness  to 
the  outer  rings,  or  the  increased  space  in  the  outer  rings  can  be 
partly  filled  with  pieces  of  ordinary  slate,  which  answer  well 
the  purpose. 

163.  The  strength  of  brick  piers  or  arches  can  be  materially 
increased  by  the  use  of  ordinary  hoop  iron,  bent  into  the  joints 
and  under  and  over  the  bricks  :  it  is  easily  and  simply  applied, 
economical,  and  can  be  recommended  ;  will  also  strengthen 
concrete  and  cement  pipes.  Wire  netting  is  also  used. 

164.  In  the  lining  of  tunnels  which  are  arches,  the  side 
walls  or  abutments  are  continuations  of  the  arch  to  the  bottom, 
the  foot  of  the  walls  being  joined  by  inverted  arches.  Tunnels 
are  generally  lined  with  brick,  on  account  of  the  ease  with 
which  brick-work  can  be  built,  especially  in  confined  and 
cramped  positions.  Often  tunnels  are  lined  with  timber ;  this, 
however,  is  only  a  temporary  and  economical  expedient.  This 
will  be  further  alluded  to  under  the  subject  of  timber. 

165.  The  thickness  of  tunnel  arches  can  only  be  fixed  by 
empirical  rules,  based  upon  the  practice  that  has  existed  through 
the  past  ages,  as  the  condition  of  the  external  forces  are  not 
thoroughly  understood  ;  but  in  case  of  tunnels,  especially  those 
at  great  depth,  the  pressure  is  practically  uniform  and  constant, 
and  the  line  of  pressure  is  fixed  and  not  altered  by  rolling 
loads,  as  is  the  case  with  arches  built  under  ordinary  conditions. 
The  cross-section  of  a  tunnel  through  ordinary  earth  requir¬ 
ing  a  lining  is  generally  two-thirds  to  three-fourths  of  an  ellipse  ; 
in  rock  it  may  be  said  to  be  of  any  shape  most  conveniently 
excavated,  giving  ample  room  for  the  purpose  intended. 
Thickness  of  arching  varies  from  20  to  36  inches. 


84  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 

166.  Arches  are  used  largely  for  crossing  streams,  streets,, 
roads,  either  over  or  under,  of  varying  lengths  and  spans. 
The  abutments  of  arches  generally  have  wing  walls  con¬ 
structed  in  one  of  the  methods  above  described  and  for  the 
same  purposes.  These  wing  walls  are  sometimes  built  on  a 
curved  batter ;  the  principles  of  construction  are,  however, 
the  same. 

167.  The  principles  of  brick  arches  as  to  stability  are  the 
same  as  in  stone-masonry  arches.  The  line  of  pressure  is  con¬ 
structed  in  the  same  manner,  the  depth  of  the  arch  ring  can  be 
found  by  the  formula  for  masonry  arches,  but  these  results 
should  be  increased  by  at  least  25  per  cent  ;  that  is,  if  the  for¬ 
mula  calls  for  2  feet  of  masonry,  it  should  be  at  least  2.5  to  3 
feet  thick  for  the  brick  arch,  but  this  is  generally  stated  as  so 
many  rings ;  as  the  brick  is  placed  flatwise  as  stretchers,  each 
ring  would  be  about  4^  inches  thick;  this  with  the  mortar-joints 
would  take  about  8  rings  or  courses. 

168.  Many  large  arches  have  been  built  of  brick,  but  as  a 
rule  it  is  used  mainly  for  very  small  arches,  stone  being  pre¬ 
ferred  wherever  it  can  be  obtained  conveniently  and  economi¬ 
cally.  It  is  not  an  unusual  plan  to  make  the  end  ring-courses 
of  ashlar  masonry,  and  between  the  two  ends  build  the  ring 
of  brick.  In  order  to  secure  a  good  bond,  three  or  more 
string-courses  of  stone  masonry  could  be  used,  the  brick  rings 
abutting  against  these  stones;  this  is  not,  however,  commonly 
resorted  to. 

169.  Brick-work  is  estimated  and  paid  for  either  by  the 
cubic  yard  or  surface  measurement.  In  the  first  case  it  is  usual 
to  state  that  so  many  brick  shall  make  a  cubic  yard  ;  this  is 
generally  estimated  at  about  five  hundred  bricks  ;  it,  however, 
depends  upon  the  size  of  the  brick  and  the  thickness  of  the 
mortar-joints.  In  the  second  a  square  or  perch  on  the  face  of 
a  wall  one  brick  thick  is  the  basis  of  estimate  ;  if  the  wall  is  two 
bricks  thick,  the  surface  is  supposed  double  :  this  on  the  face  of 
the  wall  includes  openings  either  in  part  or  entirely,  according 
to  the  agreement. 


ARCHES. 


85 


Art.  XVII. 

CONCLUSIONS. 

170.  We  may  therefore  sum  up  as  follows,  in  regard  to  the 
theories  of  the  arch,  and  their  practicable  application : 

171.  The  stability  of  the  arch,  as  of  all  structures,  depends 
upon  the  relations  existing  between  the  external  forces  or  loads 
tending  to  produce  strain,  and  the  internal  forces  or  stresses 
thus  developed  tending  to  resist  or  balance  the  external  forces. 
In  all  discussions  of  walls  or  arches  the  length  is  considered  as 
unity  ;  that  reduces  the  wall  under  consideration  to  the  value 
of  a  section  of  the  wall  or  arch  included  between  two  planes 
perpendicular  to  its  axis  at  a  unit  distance  apart,  or  simply 
equivalent  to  the  area  of  the  cross-section. 

172.  The  external  forces  to  be  considered  are  the  forces  or 
loads  acting  upon  the  structure  of  whatever  nature  they  may 
be,  including  the  weight  of  the  structure  itself,  and  the  sup¬ 
porting  forces,  whether  applied  to  the  whole  structure,  in  which 
the  supporting  pressure  is  the  resistance  of  the  foundation,  or 
whether  applied  to  any  portion  of  the  structure,  no  matter  how 
small  into  which  it  maybe  divided.  In  this  case  the  supporting 
forces  are  the  forces  or  stresses  exerted  between  the  portions 
of  the  structure  under  consideration  and  the  other  portions  in 
contact  with  them  :  the  conditions  of  equilibrium  require  that 
these  shall  balance  each  other. 

173.  A  force  is  completely  determined  when  its  point  of 
application,  its  direction,  and  its  magnitude  are  fully  known. 

174*  We  are  met,  in  deducing  any  theoretical  formula  for 
these  relations,  in  the  beginning,  with  a  great  want  of  knowledge 
as  to  either  of  these  elements  of  force,  and  in  fact  the  accurate 
determination  is  impossible. 

175-  As  a  consequence  a  great  many  suppositions  have 
been  made,  and  upon  each  supposition  some  theory  has  been 
constructed  ;  and  as  the  premises  differ  widely,  so  do  the  con¬ 
clusions. 


86 


A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


176.  We  do  not  know  the  pressure  exerted  by  earth  against 
a  retaining-wall  in  either  of  its  essential  elements,  and  less  do 
we  know  the  pressure  exerted  upon  an  arch  loaded  with  earth 
or  other  material,  and  in  addition  with  heavy  rolling  loads.  In 
arches,  however,  the  assumption  is  made  that  the  entire  load 
above  acts  vertically,  and  with  its  full  intensity  upon  the  arch 
ring  ;  this  is  certainly  on  the  side  of  safety,  eliminating  all  in¬ 
clined  or  horizontal  forces  of  any  kind.  The  second  assumption 
is  that  the  arch  ring  supports  this  entire  load  :  this  naturally 
follows  from  the  first.  The  third  assumption  is  as  to  the  point 
of  application  and  direction  of  the  thrust  or  stresses  developed 
in  the  arch  ring;  these,  however,  being  assumed,  the  magnitude 
of  the  thrust  itself  can  be  easily  determined. 

177.  Every  change,  under  the  same  external  loads  or  forces, 
in  the  direction  or  point  of  application  of  the  thrust  gives  an 
entirely  different  line  of  pressure,  upon  the  position  of  which 
the  stability  of  the  arch  is  supposed  to  depend,  and  there  may 
be  any  number  of  lines  of  pressure  ;  the  problem  of  determining 
the  true  line  is  evidently  indeterminate. 

178.  The  pressure  at  the  crown  is  supposed  to  be  horizontal, 
and  must  have  its  point  of  application  in  the  arch  ring  itself, 
and  generally  in  the  middle  third  of  its  depth. 

179.  With  these  quantities  assumed,  together  with  observing 
the  manner  in  which  arches  give  way,  we  are  enabled  to  de¬ 
termine  with  some  degree  of  approximation  the  requisite  depth 
and  thickness  of  the  arch  ring  for  any  given  form  and  size  of 
arch. 

180.  Arches  give  way  either  by  crushing  the  voussoirs,  or 
by  the  parts  sliding  on  each  other  at  some  of  the  joints,  or  by 
the  parts  rotating  either  around  the  outer  or  inner  edge  of  the 
arch  ring. 

181.  To  prevent  crushing  the  arch  stones,  the  intensity  of  the 
pressure  must  not  be  greater  than  the  strength  of  the  stone,  and 
for  safety  not  more  than  one-tenth  of  their  strength.  It  should 
be  uniformly  distributed  over  the  depth  of  the  arch  ring,  or  at 
any  rate  it  should  not  vary  from  uniformity  further  than  that 
which  can  be  represented  by  the  ordinate  of  a  right-angled  tri- 


ARCHES. 


87 


angle  whose  base  is  the  depth  of  the  arch  ring,  and  whose 
height  is  double  the  mean  pressure,  found  by  dividing  the  total 
pressure  on  any  joint  by  the  depth  of  the  joint  (which  is  the 
area  of  a  unit  of  length);  in  this  case  the  pressure  at  either  the 
intrados  or  extrados  would  be  nothing.  In  other  words,  the 
greatest  intensity  of  the  pressure  at  any  point  must  not  exceed 
two  times  the  mean  pressure,  supposing  the  total  pressure  to 
be  uniformly  distributed. 

182.  When  sliding  takes  place,  unless  caused  by  settlement 
of  one  of  the  abutments,  it  will  generally  occur  at  four  joints 
of  the  arch  ring,  splitting  it  into  five  parts:  in  flat  arches  the 
upper  parts  sliding  downwards  and  two  parts  on  either  side 
sliding  outwards;  in  pointed  arches  the  upper  part  sliding 
upwards  and  the  other  two  sliding  inwards.  To  avoid  this  the 
resultant  pressure  at  any  joint  should  not  make  with  the  nor¬ 
mal  to  the  joint  an  angle  greater  than  the  angle  of  repose. 
Radiating  joints  will  generally  fulfil  this  condition  ;  if  not,  the 
direction  of  the  joints  can  be  easily  changed. 

183.  When  arches  give  way  by  rotating  or  overturning 
around  any  joint  it  will  generally  occur  at  five  points, — one  at 
the  crown,  two  at  some  point  between  the  crown  and  the 
springing  and  two  at  the  springing, — dividing  the  arch  ring  into 
four  parts  ;  in  flat  arches  the  two  upper  parts  falling  inwards  or 
downwards,  and  the  two  lower  parts  outwards,  and  in  pointed 
arches  the  reverse.  This  will  be  prevented  by  confining  the 
line  of  pressure  within  the  arch  ring,  and  for  perfect  safety 
within  the  middle  third.  Wherever  the  arch  ring  opens  is  a 
joint  of  rupture,  but  we  generally  speak  of  the  joints  of  rupture 
as  applying  to  the  joints  on  either  side  of  the  crown,  between 
the  springing  and  the  crown.  The  exact  position  of  the  joint 
of  rupture  cannot  be  determined,  but  is  supposed  to  be  be¬ 
tween  those  joints  that  make  an  angle  from  30  to  45  degrees 
with  the  horizontal. 

184.  The  general  modes  of  determining  the  line  of  pressure 
graphically  have  been  explained. 

185.  While  building  the  arch  ring  it  must  be  supported  by 
a  frame  called  the  centre ;  this  is  generally  made  of  timber. 


88 


A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


For  small  arches  it  consists  of  an  arch  rib  composed  of  two  or 
more  layers  of  plank,  cut  into  short  pieces  of  from  5  to  6  feet, 
so  that  when  cut  to  the  form  of  the  arch  they  will  be  about  12 
inches  deep  at  centre  and  8  inches  deep  at  ends,  with  radial 
ends;  these  are  bolted  together  so  as  to  break  joints;  iron 
straps  are  generally  placed  over  the  joints  and  bolted.  The 
upper  surface  is  cut  accurately  to  the  form  of  the  curve  ;  this  rib 
is  connected  with  a  tie-beam,  which  is  generally  two  pieces  of 
plank  bolted  to  the  rib,  from  which  springs  one  or  more  ver¬ 
tical  and  radiating  struts  to  support  the  rib.  These  frames 
rest  on  vertical  supports ;  which  are  generally  capped  with  tim¬ 
ber,  and  between  the  tie-beam  of  the  rib  and  the  cap  of  the 
supports  queen  or  double  wedges  are  driven  so  as  to  bring  the 
arch  accurately  to  its  proper  position.  These  frames  are 
placed  at  short  intervals,  depending  upon  the  size  of  the  arch 
and  the  strength  of  the  frames.  Over  these  frames  and  per¬ 
pendicular  to  them  are  placed  scantlings  or  laggings,  so  as  to 
form  a  close  sheeting  to  support  the  arch  stones.  In  very 
large  arches  the  centres  are  composed  of  strong  timber  bow¬ 
string  girders,  supported  and  braced  by  as  many  direct  sup¬ 
ports  as  practicable.  It  is  necessary  that  the  ribs  of  these 
frames  should  be  practically  rigid  or  unyielding,  as  a  small 
yield  or  spring  anywhere  might  result  injuriously.  The  stones 
do  not  commence  to  bear  on  the  centre  until  the  joint  is  reached 
at  which  the  stones  would  begin  to  slide  on  each  other,  and  in¬ 
creasing  then  in  a  rapid  ratio  to  the  crown,  and  only  becomes 
self-supporting  when  the  key-stones  are  put  in  position. 

186.  A  good  part  of  the  arch,  from  the  springing  on  each 
side,  can  be  built  without  the  aid  of  centres,  and  by  a  liberal 
use  of  hoop  iron,  especially  in  brick  arches,  no  centres  need  be 
used  at  all.  Centres  are  not  removed  until  the  mortar  has 
had  time  to  dry. 


BOX  CULVERTS. 


89 


Article  XVIII. 

BOX  CULVERTS. 

187.  There  is  another  structure,  very  small,  and  seemingly 
so  unimportant  that  it  is  scarcely  ever  noticed,  but  at  the  same 
time  used  largely:  this  is  the  box  culvert,  which  can  be  built 
of  stone,  brick,  or  timber,  and  is  used  to  carry  small  streams 
under  embankments.  It  consists  essentially  of  two  walls,  1,  2, 
or  3  feet  apart,  generally  1  \  to  2  feet  thick  and  covered  over 
the  top  with  large  flat  stones ;  the  height  of  the  walls  vary  be¬ 
tween  2  and  5  feet  high  ;  if  larger  than  the  above  size  should 
be  required  to  carry  off  the  water,  it  is  usually  built  double, 
that  is,  two  side  walls  and  a  middle  wall,  mainly  on  account  of 
the  difficulty  in  securing  such  large  capping-stones  as  would  be 
required.  The  ends  can  be  left  rough  or  neatly  finished,  and 
have  small  wing  walls  :  its  length  in  this  case  will  be  a  little 
shorter  than  the  total  width  of  the  embankment  at  the  bottom  ; 
if  no  wing-walls  and  no  spandrels  are  used,  the  total  length 
must  be  equal  to  or  greater  than  that  width.  At  the  ends  an 
apron  wall  is  built ;  a  trench  is  dug  two  or  more  feet  deeper 
than  the  foundation  of  the  side  walls,  and  perpendicular  to  the 
axis  of  the  culvert,  and  built  up  with  masonry,  the  object  of 
which  is  to  prevent  the  undermining  action  of  the  stream,  the 
bottom  of  the  culvert  is  generally  paved  with  small  stones;  the 
embankment  is  then  built  over  the  culvert.  Arches  for  the 
same  purpose  commonly  have  the  apron  walls,  and  are  paved 
in  the  same  manner.  Arches  are  used  when  an  opening  more 
than  5  feet  wide  is  required  to  pass  the  water. 

188.  In  filling  over  arches  and  culverts,  special  care  must 
be  taken  not  to  endanger  the  stability  by  shocks ;  the  earth 
should  be  deposited  on  both  sides  at  the  same  time,  thrown 
by  shovels,  and  should  be  rammed  in  place  ;  this  should  be 
done  for  about  10  feet  on  both  sides  and  on  top,  after  which 
the  earth  can  be  dumped  on  in  the  usual  manner.  This  pre¬ 
caution  is  quite  an  important  one,  and  should  not  be  neg¬ 
lected. 


9o 


A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


189.  For  the  purpose  of  carrying  small  streams  under 
embankments,  terra-cotta  pipes,  owing  to  their  comparative 
cheapness,  are  now  largely  used  in  sizes  from  6  inches  to  2  feet 
A  special  bed  of  sand  or  fine  earth  should  first  be  prepared  to 
receive  the  pipes,  otherwise  they  are  likely  to  be  broken  or 
distorted  by  the  weight  ;  the  earth  over  and  around  them 
should  be  carefully  placed  and  packed  ;  the  ends  should  gen¬ 
erally  rest  in  masonry  head  walls  of  some  kind  ;  the  joints 
should  be  filled  with  cement. 

GENERAL  PRINCIPLES. 

Certain  general  rules  or  principles  should  be  followed  in 
constructing  masonry  structures. 

190.  The  courses  of  masonry  should,  in  general,  be  laid 
perpendicular  to  the  resultant  pressure;  in  ordinary  cases  hori¬ 
zontal  courses  will  satisfy  this  condition  well  enough. 

191.  The  vertical  joints  should  not  be  continuous,  but 
should  be  broken  from  course  to  course  by  overlapping  the 
stones  by  a  distance  equal  to  the  depth  of  the  course,  which 
in  ashlar  should  not  be  less  than  one  foot.  This  is  known  as 
the  bond.  A  sufficient  number  of  headers  should  be  used  to 
tie  the  wall  together  transversely,  and  these  should  be  placed 
as  nearly  over  the  centre  of  the  stretchers  below  as  possible. 

192.  All  joints  and  spaces  between  the  stones  should  be 
fully  filled  with  mortar. 

193.  Stratified  stones  should  be  laid  on  their  natural  beds, 
— generally  known  as  the  quarry  bed. 

194.  The  surfaces  of  porous  stones  should  be  moistened  be¬ 
fore  being  placed  in  position;  this  is  essential  with  sandstone 
and  brick.  For  appearance’  sake,  the  largest  stones  should 
be  placed  near  the  bottom,  the  thickness  of  the  courses  gradu¬ 
ally  decreasing  towards  the  top.  A  good  rule  for  the  lengths 
of  the  headers  is  to  make  them  equal  to  ^  the  thickness  of  the 
wall  at  the  point  where  placed,  provided  that  they  will  not  ex¬ 
ceed  6  feet  in  length.  All  the  above  except  the  last  applies 
equally  to  brick-work. 

195.  Good  brick  should  be  regular  in  shape,  opposite  plane 


CEMENT. 


9i 


surfaces  parallel  to  each  other,  and  all  angles  right  angles  ; 
should  give  a  clear  ringing  sound  when  struck;  should  show  on 
a  broken  surface  a  hard,  compact  and  uniform  structure ;  and 
should  not  absorb  more  than  one  fifteenth  of  their  weight  of 
water. 

Article  XIX. 

CEMENT. 

196.  The  term  Hydraulic  Cement,  or  simply  Cement,  is 
applied  to  those  substances,  whether  natural  or  artificial,  which, 
when  calcined  and  ground  into  powder  and  mixed  with  water, 
form  a  paste  possessing  the  property  of  hardening  under  water. 
There  is  almost  an  infinite  variety  of  these  stones,  found  in 
layers  or  strata  of  different  thicknesses,  in  the  States  of  New 
York,  Pennsylvania,  Maryland,  Virginia,  Tennessee,  and  other 
States.  These  different  strata,  whether  found  in  the  same 
locality  overlying  each  other,  or  in  the  same  neighborhood,  or 
in  the  different  States,  are  found  to  be  of  different  composi¬ 
tion,  and  when  treated  in  the  same  way  yield  products  dif¬ 
fering  in  a  marked  degree  in  regard  to  their  hydraulic  activity 
or  rapidity  of  setting,  and  in  hydraulic  energy,  or  that  property 
by  which,  whether  they  set  rapidly  or  slowly,  they  attain  a 
great  and  progressively  increasing  strength.  Some  take  an 
initial  set  rapidly,  but  seem  to  increase  in  strength  and  hard¬ 
ness  very  slowly  afterwards ;  others  are  slower  in  taking  the 
initial  set,  but  show  a  more  regular  and  continuous  increase  in 
hardness  than  the  first,  and  ultimately  are  far  better.  This 
difference  is  due  not  only  to  variations  in  composition,  but 
also  in  a  large  degree  to  the  degree  of  heat  and  the  time  con¬ 
sumed  in  the  burning,  so  much  so,  that  some  of  them  only 
partly  calcined  possess  little  or  no  hydraulic  energy ;  others,  if 
at  all  overburnt,  lose  this  property.  In  order,  therefore,  to 
produce  a  cement  that  will  neither  have  too  great  nor  too 
little  hydraulic  activity,  and  therefore  better  suited  for  ordinary 
purposes,  the  manufacturers  mix  the  different  grades  of  crude 
material,  and  obtain  a  product  which  is  a  more  or  less  homo- 


92 


A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


geneous  mixture  of  the  several  grades.  This  may  explain  the 
want  of  uniformity  so  often  found  in  the  same  brand  of  cement 
obtained  at  different  times.  Frequently,  in  works  of  great 
magnitude  continuing  through  a  period  of  years,  requiring 
large  quantities  of  cement,  some  cargoes  show  a  marked  differ¬ 
ence  in  the  time  of  setting ;  even  with  every  precaution  an 
ordinary  batch  of  mortar  will  show  signs  of  stiffening  and  set¬ 
ting  before  it  can  be  used,  resulting  often,  under  strict  inspec¬ 
tion,  in  much  waste,  and  again  so  slow  in  setting  as  to  arouse 
suspicion  that  the  entire  cargo  of  cement  is  of  an  inferior  grade. 
The  writer  has  often  seen  some  cements  so  quick  in  setting 
that  they  would  stiffen  to  such  a  degree  in  the  short  interval 
required  to  lift  the  boxes  and  land  them  on  the  top  of  the  pier, 
that  it  would  be  necessary  to  work  the  mortar  again  before 
using,  the  interval  being  not  over  5  to  10  minutes,  and  in  other 
cases  after  standing  all  night  little  or  no  appreciable  change 
had  taken  place.  The  above  cements  are  generally  called  the 
light  quick-setting  cements,  weigh  about  300  lbs.  per  barrel, 
and  set  in  5  minutes  to 4  or  5  hours;  are  calcined  at  a  moderate 
temperature,  when  7  days  old,  6  days  in  water,  should  have  a 
tensile  strength  of  not  less  than  60  lbs.  per  square  inch  and 
contain  from  20  to  40  per  cent  of  clay.  These  cements  are 
generally  known  as  the  Rosendales,  the  Cumberland,  Round 
Top,  James  River,  Louisville,  etc.  ;  are  found,  respectively,  in 
New  York,  Maryland,  Virginia,  and  Kentucky. 

19 7.  What  are  known  as  the  heavy,  slow-setting  cements 
are  almost  entirely  artificial  products,  and  are  commonly  known 
as  Portland  cements,  such  as  the  German,  English,  French,  and 
American  brands.  They  are  composed  of  pure  clay  and  lime 
containing  from  20  to  25  per  cent  of  clay,  are  calcined  at  a  very 
high  temperature,  weigh  about  400  lbs.  per  barrel,  and  should 
have  a  tensile  strength  of  180  lbs.  when  7  days  old,  6  days  in 
water.  These  cements  possess  both  great  hydraulic  activity 
and  energy,  and  are  far  superior  in  every  respect  to  the  natural 
cements. 

198.  The  temperature  of  the  air  and  water  have  much  to 
do  with  the  setting  of  cements,  and  affect  them  in  different 


CEMENT. 


93 


degrees.  As  illustrating  this,  the  writer  noticed  that  Alsen’s 
German  Portland  cement  mortar  was  being  delivered  to  the 
crib  smoking  and  hot,  and  on  being  emptied  from  the  box  the 
rr  ass  fell  to  pieces  as  damp  sand  would  do,  and  showing  evi¬ 
dently  an  initial  set.  Upon  inquiring  into  the  cause,  he  found 
that  for  some  reason  hot  water  was  being  delivered  through 
the  pump ;  this  happened  on  several  occasions, — the  time 
of  passing  from  the  mixer  to  the  crib  could  not  have  been 
over  3  to  5  minutes, — and  caused  some  considerable  waste. 
This  was  ascribed  by  the  men  at  first  to  what  they  called  hot 
barrels,  but  was  found  to  be  due  to  the  use  of  hot  water. 
Concrete  in  the  working  chambers  of  caissons  will  set  almost 
immediately,  the  temperature  ranging  from  8o°  to  90°  Fahr., 
or  more,  and  after  standing  24  hours  will  require  blasting  to 
remove  it.  This  was  done  in  a  caisson  at  the  Schuylkill  River. 
The  result  is  the  same  whether  mixed  with  hot  water,  immersed 
in  hot  water,  or  placed  in  a  hot  atmosphere.  Some  engineers 
require  the  cement  and  broken  stone  to  be  carried  separately 
into  the  caisson  and  mixed  below  on  this  account.  It  may 
have  some  advantages,  but  the  disadvantages  would  seem  to 
be  greater.  The  ingredients  are  not  likely  to  be  mixed  as  care¬ 
fully  or  as  thoroughly,  they  would  be  exposed  for  a  longer 
time  to  the  hot  air  of  the  working  chamber,  which  is  frequently 
very  dry,  and  in  addition  will  be  more  expensive.  The  writer 
always  required  one  or  two  bucketsful  of  water  to  be  poured 
into  the  supply  shaft  just  before  throwing  in  the  concrete.  This 
concrete  was  never  mixed  until  a  signal  was  given  from  below 
that  they  were  ready  for  the  concrete;  the  sand  and  cement 
were  ready  mixed,  broken  stone  collected;  it  would  then  take 
only  a  few  minutes  to  make  the  concrete,  which  was  thrown  im¬ 
mediately  into  the  shaft.  The  compressed  air  passes  rapidly  into 
the  shaft,  the  concrete  drops  on  a  platform  and  is  immediately 
wheeled  by  barrows  or  shovelled  into  its  place,  deposited  and 
rammed,  the  entire  time  consumed  being  not  over  10  to  15 
minutes.  In  this  connection  Gen.  Gillmore  mentions,  page8i, 
that  of  two  samples  of  cement  paste,  which  set  in  90°  Fahr.  in 
and  4  minutes,  required  at  65°,  6  and  17  min.,  and  at  350,  39 


94 


A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


and  82  min.,  respectively,  to  get  the  same  set ;  i.e.,  fora  depres¬ 
sion  of  temperature  from  90°  to  350  =  550,  the  delay  in  setting 
to  the  same  extent  was  37^-  minutes  in  one  case  and  1  hour 
and  18  minutes  in  the  other,  and  concludes  that  the  presence 
of  an  excess  of  caustic  lime  in  some  of  the  varieties  of  cement 
causes  them  to  be  quick-setting,  due  to  the  heat  developed  in 
bringing  this  lime  to  a  state  of  hydrate. 

199.  What  is  meant  by  a  “set  ”  is  not  well  defined.  Gen. 
Gillmore  defines  it  as  a  state  of  the  paste  in  which  it  will  not 
change  its  form  without  fracture,  or  when  it  has  entirely  lost 
its  plasticity  this  is  evidently  a  vague  and  uncertain  standard 
of  comparison.  Another  test  of  the  setting  is  the  time  that  is 
requisite  for  the  mortar  to  bear  a  small  wire  loaded  with  a  cer¬ 
tain  weight,— a  Jg-inch  wire  loaded  with  \  pound,  and  a  ^-inch 
wire  loaded  with  I  pound  ;  and  when  the  mortar  will  support 
these  weights  without  indentation  or  depression  it  is  said  to 
have  “  set.”  The  latter  is  purely  a  surface  test,  and  as  the  tem¬ 
perature  of  the  air  and  water  play  such  an  important  part  in 
determining  the  time  of  taking  a  set,  its  value  is  only  for 
comparison  of  the  hydraulic  activity  of  two  or  more  different 
brands,  and  would  vary  greatly  according  to  the  time  of  the 
year,  and  can  be  of  but  little  practical  value,  as  the  cement 
may  be  tested  at  one  time  and  used  at  another.  If  the  tensile 
strength  is  taken  as  the  standard,  the  Portland  cements  should 
be  called  the  quick-setting,  and  the  ordinary  cements  slow-set¬ 
ting.  In  practice,  however,  the  distinction  is  not  of  very  great 
importance,  as  most  cements,  when  mixed  in  small  quantities, 
afford  ample  time  for  using  before  any  harmful  change  takes 
place  in  the  mortar;  but  this  is  not,  however,  universal.  A 
quick-setting  cement  has  some  advantages  when  exposed  to 
immediate  causes  of  deterioration  or  destruction,  as  when  used 
in  sea-water,  works  under  such  circumstances  being  constructed 
at  low  tide,  and  shortly  flooded  by  high  tide  ;  under  such  cir¬ 
cumstances  a  slow-setting  cement  could  be  used  and  faced,  or 
protected  by  an  inferior  but  very  quick-setting  cement. 

200.  Quick-lime  is  ordinarily  slaked  by  pouring  the  entire 
quantity  of  water  necessary  on  the  lime  at  one  time, — about 


CEMENT. 


95 


two  or  three  times  the  volume  of  the  quick-lime.  After  slak¬ 
ing  has  commenced  an  addition  of  cold  water  is  inj urious.  Good 
lime  should  not  require  stirring  or  the  breaking  of  lumps  dur¬ 
ing  slaking.  The  same  method  is  adopted  in  adding  water  to 
cement,  but  as  this  is  mixed  immediately  before  using,  it  is 
important  not  to  use  too  much  water,  as  the  mortar  will  be  too 
soft,  and  to  avoid  this  the  general  practice  is  to  mix  the  mor¬ 
tar  at  first  rather  dry,  and  then  temper  it  with  a  small  addition 
of  water,  to  the  proper  consistency.  This  is  preferable  to  mak¬ 
ing  it  too  soft  and  wet  at  first,  and  then  adding  dry  cement 
powder  to  bring  it  to  a  proper  plastic  condition. 

The  quantity  of  water  necessary  in  mixing  cement  varies 
materially  with  the  kind  of  cement  used,  the  condition  of  the 
weather  as  to  heat,  moisture,  or  dryness,  the  age  of  the  cement, 
whether  dry  or  moist  sand  is  used,  and  whether  the  broken 
stone  is  moist  or  dry.  To  form  a  paste  of  cement  mortar  of 
ordinary  consistency  i  bbl.  of  cement  will  require  about  ^  of  a 
barrel  of  water,  but  when  sand  is  used  more  water  will  be  re¬ 
quired  ;  but  this  excess  should  be  added  in  small  quantities,  as 
at  a  certain  plastic  state  even  small  quantities  of  water  will 
make  the  mortar  too  soft.  An  ordinary  cement  barrel  con¬ 
tains  3f  cubic  feet  of  space,  and  about  5  cubic  feet  of  loose 
cement  can  be  packed  in  the  barrels.  Mixed  in  the  above 
proportions  there  will  result  about  §  of  a  barrel  of  paste.  In 
some  cases  the  sand  is  mixed  with  the  paste,  but  the  general 
practice  is  to  first  mix  the  sand  and  cement  dry.  These  should 
be  turned  over  and  over  until  it  has  a  uniform  appearance. 
When  carelessly  mixed,  patches  or  layers  of  cement  without 
sand  and  sand  without  cement  are  readily  seen,  and  if  water 
is  added  in  this  condition  it  will  be  impossible  to  secure  a 
homogeneous  mortar.  1  barrel  of  cement,  2  barrels  of  sand, 
will  make  from  8  to  8J  cubic  feet  of  mortar,  which  will  ordi¬ 
narily  be  sufficient  for  laying  I  cubic  yard  of  brick-work  or 
hammer-dressed  rubble- work,  and  in  making  1  cubic  yard  of 
concrete.  And  I  barrel  of  cement,  3  barrels  of  sand,  will 
make  about  12  cubic  feet  of  mortar,  sufficient  for  i-J  cubic 
yards  of  ordinary  masonry  and  concrete.  Rough  rubble  will 


96  A  PRACTICAL  TREAT/ SE  ON  FOUNDATIONS. 


require  from  1 1  to  12  cubic  feet  of  mortar  per  cubic  yard,  or  if- 
barrels  of  cement  per  cubic  yard  ;  if  mixed,  1  cement,  2  sand, 
or  1  barrel  of  cement  ;  if  mixed,  1  cement,  3  sand.  1  barrel  of 
cement  should  be  sufficient  for  if  cubic  yards  of  good  ashlar 
masonry.  For  quick-lime  mortar  only  about  f  of  the  above 
is  necessary,  f  barrel  of  lime  being  equivalent  to  1  barrel  of 
cement.  The  above  quantities  are  fair  approximations,  and 
will  serve  as  a  good  basis  for  estimating  the  number  of  barrels 
of  cement  used  in  any  proposed  construction  of  masonry, 
whether  ashlar,  brick,  concrete,  or  rubble,  and  consequently 
the  cost  of  the  same  per  cubic  yard. 

201.  Mr  Trautwine,  in  edition  of  1888,  gives  the  following: 

Crushing  strength, 

7  days  old,  Tons  per 

6  days  in  water,  sq.  ft. 

in  lbs.  per  sq.  in. 

1100  to  2500  71  to  154. 

1100  “  2500  71  “  154 

250  “  450  16  “  29 

The  practical  engineer  will  rarely  be  able  to  do  more  than 
determine  the  tensile  strength,  as  described  in  another  para¬ 
graph.  The  above  results  seem  to  be  low,  and  any  cement  not 
showing  a  strength  equal  to  the  inferior  limits  of  170  and  40 
lbs.  as  above  should  be  rejected.  For  Portland  cement  it  is 
not  unusual  to  specify  that  at  the  age  of  7  days,  6  days  in  water 
the  tensile  strength  should  be  at  least  300  lbs.  per  square  inch, 
and  sometimes  as  high  as  500  lbs.,  and  for  the  American  brands, 
such  as  the  Rosendale,  Louisville,  etc.,  should  certainly  never 
fall  below  40  lbs.,  and  should  be  required  to  stand  the  superior 
limit  of  70  lbs.  in  the  time  specified. 

Article  XX. 

MORTAR. 

202.  MORTAR  is  a  mixture  of  lime  or  cement,  sand,  and 
water  in  certain  more  or  less  definite  proportions.  It  is  usual 
to  prescribe  the  proportions  of  lime  or  cement  to  that  of  sand 


Tensile  strength, 
7  days  old, 

6  days  in  water, 
in  lbs.  per  sq.  in. 

Portland  Cement  (neat),  ...  170  to  370 

“  “  (2  sand),  .  22  “  126 

American  “  (neat),  ...  40  “  70 

“  “  (2  sand),  .  22 


MORTAR. 


97 


as  I  to  i,  I  to  2,  i  to  3,  etc,  the  proportion  of  sand  depending 
upon  the  kind  of  cement  and  nature  and  importance  of  the 
work.  Sometimes  these  proportions  mean  by  volume,  some¬ 
times  by  weight,  the  amount  of  water  being  regulated  somewhat 
arbitrarily.  For  ordinary  walls  quick-lime  is  generally  used  ; 
for  more  important  works  cement :  sometimes  the  two  are 
mixed.  The  volume  of  mortar  varies  from  one  eighth  to  one 
third  the  volume  of  stone,  the  larger  limit  in  concrete  and  rub¬ 
ble.  It  is  stated  on  good  authority  that  one  volume  of  lime 
paste  can  be  mixed  with  one  volume  of  cement  paste  without 
material  loss  of  strength,  and  that  a  mixture  of  lime  paste  equal 
to  one  half  or  three  fourths  that  of  the  cement  paste  produces 
no  appreciable  injury,  and  is  suitable  for  concrete  when  under 
water,  and  even  better  on  many  accounts.  When  not  imme¬ 
diately  immersed,  it  has  the  advantage  of  making  some  quick¬ 
setting  cement  slower  in  setting,  and  is  certainly  economical. 
The  practice,  however,  differs,  and  it  can  be  safely  said  that 
there  is  a  very  decided  prejudice  against  this  mixture. 

20 3.  Quick-lime  is  the  product  resulting  from  burning  lime¬ 
stone  in  a  proper  constructed  kiln,  the  heat  driving  off  the  car¬ 
bonic  acid,  leaving  white  lumps  and  powder,  which  is  the  lime 
of  commerce ;  this  when  mixed  with  water  undergoes  “  slak¬ 
ing,”  a  chemical  action  being  set  up,  the  water  combining  with 
the  lime,  which  in  the  process  falls  to  powder  and  results  in  a 
stiff  white  paste.  The  volume  swells  and  great  heat  is  developed. 
The  perfectness  of  this  process  is  probably  the  best  and  surest 
test  of  the  quality  ;  the  presence  of  lumps  or  cores  that  will  not 
slake  means  either  an  inferior  quality  of  lime  or  insufficient 
burning,  and  results  in  great  waste. 

204.  The  proportions  of  sand  and  lime  vary  from  3  to  6  vol¬ 
umes  of  sand  to  one  volume  of  quick-lime  ;  the  mortar  thus 
procured  is  used  extensively  in  walls  of  houses,  and  even  in 
more  important  structures,  but  should  never  be  used  under 
ground  or  under  water,  as  it  will  not  attain  any  great  degree  of 
hardness  under  these  conditions,  if  it  hardens  at  all ;  that  this 
is  often  done,  only  proves  that  our  structures  are  many  times 
stronger  than  required.  Lime  mortar  should  never  be  used  in 


98  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 

concrete  in  large  masses,  as  it  may  be  doubted  if  the  interior 
of  the  mass  will  ever  harden,  as  lime  mortar  sets  alone  by  ab¬ 
sorbing  carbonic  acid  from  the  air,  returning  to  the  condition 
of  a  carbonate  of  lime,  the  cemented  material  then  becoming 
an  artificial  sandstone.  Cement  mortars  set  by  chemical 
action,  and  probably  throughout  the  entire  mass  at  the  same 
time. 

205.  The  ordinary  limestone  contains  other  ingredients  such 
as  silica,  alumina,  magnesia,  in  such  small  proportions,  say  not 
to  exceed  10  per  cent,  that  they  exert  no  beneficial  or  harm¬ 
ful  influence  worth  considering,  but  when  we  find  limestone 
which  contains  these  elements  in  proportions  between  10  to  60 
per  cent,  we  find  stones  that  possess  peculiar  properties,  and  of 
immense  value  and  importance.  The  writer,  in  what  he  has 
said  on  this  subject  and  what  follows,  does  not  propose  to  enter 
into  the  refinements  of  this  subject,  but  to  present  a  few  facts 
which  occur  to  him,  of  the  greatest  practical  value.  Those  who 
desire  can  find  a  great  deal  of  most  interesting  and  valuable 
information,  and  doubtless  the  best  available  (up  to  time  of 
publication,  1872)  in  Gillmore’s  book  on  limes,  hydraulic  cem¬ 
ents,  and  mortars. 

206.  Cement  stones  exist  in  all  states,  from  the  very  slightly 
hydraulic  to  the  intensely  so,  culminating  in  the  Portland  cem¬ 
ents,  which  are  artificial  products  resulting  from  the  mixture 
of  pure  clay  and  pure  limestone  in  certain  definite  proportions 
determined  by  experiment,  thoroughly  mixed  and  calcined  at 
a  very  high  temperature,  then  ground  to  a  fine  powder.  Port¬ 
land  cement  is  of  course  the  best  in  every  respect,  but  it  is 
very  expensive,  and  consequently  only  used  in  certain  special 
structures  of  great  magnitude  and  importance.  For  ordinary 
purposes  requiring  the  use  of  hydraulic  cement,  the  natural 
stone  possessing  hydraulic  properties  is  calcined  in  a  kiln, 
thoroughly  ground  and  barrelled  for  use.  These  natural  stones 
are  found  in  many  parts  of  the  Middle  States,  each  varying  in 
some  respect  ;  some  very-slow  setting,  some  very  quick-setting, 
and  other  medium  ;  generally  when  ground  of  a  mouse  color, 
but  some  decidedly  yellow.  It  cannot  be  denied  that  some  very 


MORTAR. 


99 


inferior  grades  are  put  upon  the  market,  and  without  careful 
testing,  are  used,  but  as  a  rule  the  standard  companies  cannot 
afford  to  take  the  risk  of  such  conduct,  and  can  be  relied  on  in 
the  main. 

207.  As  to  different  cement  brands,  we  must  first  be  guided 
by  tests  that  have  been  made,  and  select  that  one  which  seems 
best  suited  for  the  purpose  in  view,  and  in  addition  simple  test 
should  be  made  on  delivery.  All  broken  barrels  should  be  re¬ 
jected,  especially  if  it  is  to  be  stored  for  any  length  of  time,  as 
by  exposure  to  the  air  it  will  take  a  set  and  be  useless,  and  as 
it  also  results  in  waste.  On  opening  barrels  small  portions 
found  to  be  set  will  not  necessarily  indicate  that  the  balance 
is  ruined  ;  but  it  excites  suspicion  of  undue  age  or  exposure,  and 
means  so  much  waste. 

208.  It  is  well  to  test  a  fair  number  of  barrels  by  inserting 
the  hand  into  the  mass  of  cement,  principally  to  determine  the 
fineness  to  which  it  is  ground,  as  a  little  experience  will  enable 
you  to  measure  the  sensitiveness  of  the  touch  ;  but  this  can 
easily  be  verified  by  obtaining  a  sieve  with  small  meshes,  num¬ 
bered  according  to  the  number  of  meshes  to  the  square  inch. 
A  No.  60  sieve  would  contain  3600  meshes  to  the  square  inch, 
a  number  50  sieve  2500  to  the  square  inch;  this  last  ought  to 
pass  the  entire  quantity,  except  a  small  per  cent ;  the  coarser 
particles  add  nothing  to  the  value  of  the  cement,  and  amount 
to  so  much  waste.  In  addition,  small  cakes  of  mortar  made 
with  good  sand  and  cement  in  proportion  used  on  the  work 
will  give  a  very  good  idea  of  its  setting  qualities  either  in  air 
or  water. 

209.  Where  large  quantities  of  cement  are  being  delivered 
and  used  more  or  less  rapidly,  the  above  are  the  only  tests 
practicable,  and  using  the  standard  brands,  it  will  determine 
practically  whether  the  cargo  under  consideration  is  up  to  the 
standard.  A  medium,  slow-setting  cement  is  preferable,  other 
things  being  equal,  to  a  very  quick-setting  cement,  as  in  large 
works  the  mortar  will  have  to  stand  for  some  little  time  before 
being  used  entirely  ;  this  should  be  avoided  as  far  as  possible. 

210.  In  general,  you  can  rely  on  the  Portland  cements, 


IOO 


A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


German,  English,  or  American  brands,  for  any  purpose  ;  the 
Rosendale,  Norton,  and  Hoffman  brands,  the  Louisville,  James 
River,  Va.,  and  many  other  brands  have  generally  given  entire 
satisfaction  in  the  writer’s  experience.  The  writer  has  had 
but  little  experience  with  lime  and  cement  mixed,  but  experi¬ 
ment  as  well  as  experience  seem  to  authorize  the  use  of  it  in 
the  proportions  above  mentioned,  and  it  certainly  possesses 
the  advantage  of  economy. 

211.  There  have  been  comparatively  few  experiments  made 
to  determine  the  resistance  to  crushing  of  mortars  when 
mixed  with  sand,  and  it  has  been  the  practice  to  determine 
the  tensile  strength  of  the  many  brands  of  cement,  and  from 
these  results  the  crushing  strength  is,  to  a  great  extent,  in¬ 
ferred,  on  the  supposition  that  a  high  tensile  strength  indicates 
a  high  crushing  strength.  Many  and  varied  experiments  have 
been  made  in  this  manner. 

212.  Briquettes  of  neat  cement,  and  mixed  with  varying 
proportions  of  sand,  i  to  I,  i  to  2,  I  to  3,  have  been  tested. 
These  briquettes  have  rounded  ends,  connected  by  a  square 
prism  of  exactly  1  square  inch  cross-section ;  they  are  so 
formed  that  they  will  break  at  some  point  between  the  heads, 
and  not  at  the  junction  of  the  head  and  neck.  Brass  moulds  of 
the  proper  size  and  shape  are  made,  and  the  mortar  pressed 
into  the  mould  and  allowed  to  get  an  initial  set.  The  mould  is 
then  removed,  and  the  test  is  made  on  several  briquettes 
made  from  the  same  batch  of  mortar,  at  intervals  of  1,2,  3,  7, 
10  and  more  days.  The  testing  machines  are  provided  with 
nippers  or  clutches,  and  so  adjusted  as  to  take  hold  of  the  head 
exactly  at  the  junction  of  the  head  and  neck,  and  so  situated 
as  to  make  the  pull  exactly  in  a  straight  line,  with  no  twisting 
or  jerking,  the  power  being  slowly  and  gradually  increased 
until  the  briquette  is  pulled  apart;  and  it  is  generally  specified 
on  important  works  that  the  tensile  strength  should  be  so 
many  pounds  per  square  inch,  in  so  many  days  after  mixing. 
Machines  and  moulds  are  specially  made  for  this  purpose,  and 
it  would  be  useless  to  enter  into  any  detailed  description,  as 
they  can  be  more  easily  purchased  than  made  to  order  ;  and 


MORTAR. 


IOI 


in  any  event,  unless  the  work  is  of  very  great  importance,  and 
cement  is  used  in  large  quantities,  the  simple  tests  above 
alluded  to  will  be  satisfactory. 

213.  Pure,  rich,  or  fat  lime  mortar  hardens  slowly  in  air  by 
the  absorption  of  carbonic  acid  gas,  when  used  in  compara¬ 
tively  thin  walls,  but  it  may  never  harden  at  all  in  the  interior 
of  large  masses.  It  will  not  set  or  harden  under  water  or  in 
wet  soils,  and  should  therefore  never  be  used  under  water 
under  any  circumstances,  and  it  is  false  economy  to  use  it 
under  ground  in  damp  earths.  When  water  is  added  to  lime 
it  undergoes  the  processs  of  slaking  ;  great  heat  is  evolved  ;  it 
swells  to  2  or  3  times  its  original  bulk,  falls  to  a  powder,  and 
the  resulting  product  is  a  hydrate  of  lime,  unctuous  or  soapy 
to  the  touch,  and  forms  a  stiff  paste.  Lime  should  not  be 
exposed  to  the  air,  as  it  will  in  time  become  air  slaked,  and 
materially  injured.  But  when  mixed  in  paste  with  sand,  it  is 
considered  better  to  let  it  stand  before  using,  and  for  this 
reason  lime  mortar  is  mixed  in  large  quantities,  left  in  piles 
and  used  as  needed  after  stirring  and  tempering.  Lime  mortar 
shrinks  considerably  in  setting. 

214.  Hydraulic  limes  containing  from  10  to  20  per  cent  of 
silicates  harden  in  air  or  water,  but  somewhat  slowly  under 
water;  they  slake  to  some  extent,  but  slowly,  and  are  sup¬ 
posed  to  harden  by  chemical  action,  probably  through  the 
whole  mass  at  the  same  time,  and  shrink  but  little  in  setting. 
It  should  not  be  allowed  to  stand  any  great  length  of  time 
after  being  mixed  with  water,  as  it  will  take  an  initial  set, 
which,  when  disturbed  by  remixing,  is  supposed  to  materially 
diminish  its  ultimate  strength. 

215.  Hydraulic  cements,  or  simply  cements,  contain  from 
20  to  60  per  cent  of  silicates.  The  proportion  of  silicates  to 
the  proportion  of  carbonate  of  lime  determine  the  value  of  the 
cement.  These  vary  considerably,  and  we  have  accordingly 
the  heavy,  slow-setting  cements  on  the  one  hand.  These  are 
called  Portland  cements,  and  generally  are  manufactured,  using 
pure  clay  and  pure  lime  mixed  in  definite  proportions  deter¬ 
mined  by  experiment.  This  mixture  is  then  burned  at  a  high 


102  A  PRACTICAL  TREATISE  ON  FOUNDATIONS . 

temperature,  ground  exceedingly  fine,  carefully  packed  in  good, 
strong,  and  tight  barrels,  generally  lined  with  brown  paper,  in 
order  to  prevent  any  possible  absorption  of  moisture ;  cost 
from  2  to  2 ^  times  per  barrel  more  than  the  ordinary  cements; 
weigh  considerably  more  per  barrel,  and,  owing  to  their  ten¬ 
dency  to  set  quickly,  should  be  mixed  with  water  only  a  few 
minutes  before  using,  and  only  in  small  quantities  at  a  time. 
The  more  common  brands  are  the  German,  English,  and  Amer¬ 
ican,  all  of  which  brands  are  of  excellent  quality,  and  suitable 
for  any  purpose,  and  will  stand  two,  three,  or  more  volumes  of 
sand,  and  seem  not  to  be  injured.  These  harden  rapidly  in  air 
or  water,  and  attain  great  ultimate  strength. 

216.  The  ordinary  cements  are  obtained  from  natural 
stones,  found  in  many  parts  of  the  country,  and  are  known  as 
the  light,  quick-setting  cements;  only  weigh  about  two  thirds 
as  much,  and  take  a  very  much  greater  time  to  set,  and  do  not 
attain  more  than  about  half  as  much  ultimate  strength  as  the 
Portland  cements,  but  are  strong  enough  for  almost  any  pur¬ 
pose,  and,  owing  to  their  great  abundance  and  relatively  low 
cost,  are  used  for  all  ordinary  purposes,  and  can  be  more  con¬ 
veniently  handled,  as  they  take  more  time  to  set,  but  should  not 
be  mixed  with  water  any  great  time  before  being  used.  The 
proportion  of  sand  is  rarely  over  2  to  I  of  cement.  Some 
brands  of  these  cements  set  much  more  rapidly  than  others, — 
so  much  so  that  some  of  them  could  be  properly  called  slow- 
setting.  All  of  these  cements  set  well  in  air  and  in  water. 

217.  If  mortar,  whether  used  alone  or  in  concrete,  is  to  be 
deposited  under  water,  it  should  be  allowed  to  take  an  initial 
set,  as  otherwise  the  cement  will  almost  invariably  be  sepa¬ 
rated  from  the  sand  and  the  stone,  no  matter  how  carefully  it 
may  be  deposited.  Perhaps  the  best  mode  of  depositing  con¬ 
crete  under  water  is  to  fill  open  sacks  or  gunny  sacks  about 
two-thirds  to  three-fourths  full  of  the  concrete  or  mortar  and 
deposit  these  in  place,  arranging  them  in  courses,  where 
practicable,  header  and  stretcher  system,  and  ramming  each 
course  as  laid  ;  the  bagging  is  close  enough  not  to  allow  the 
cement  to  be  washed  out,  but  at  the  same  time  open  enough 


MOR  TAR. 


103 


to  allow  the  whole  mass  to  be  united  and  to  become  as  com¬ 
pact  as  concrete  itself.  The  writer  used  this  method  in  the 
foundation  of  a  pier  over  100  feet  high,  and  has  also  adopted 
this  plan  in  other  works  of  less  magnitude,  but  never  has  the 
result  been  satisfactory  when  deposited  under  water  in  any 
other  manner. 

218.  In  whatever  way  mortar  has  been  deposited  under 
water,  the  result  is  at  least  uncertain  ;  and  there  is  positive  evi¬ 
dence  that  in  some  cases  where  divers  have  examined  concrete 
thus  deposited  it  has  been  found  to  be  far  from  homogeneous: 
deficiency  of  sand  and  excess  of  cement  in  some  places  and  the 
reverse  in  others,  and  the  same  as  to  the  stone  and  cement. 
Some  experiments  on  a  large  scale  were  made  by  General 
Newton,  using  a  very  large  box  or  caisson  filled  with  water 
and  depositing  the  concrete  therein,  with  every  precaution 
taken  in  order  to  secure  favorable  results;  then  subsequently 
pumping  the  water  out,  and  removing  the  sides  in  order  to 
make  a  thorough  examination  as  to  its  condition.  The  mass 
was  found  to  be  far  from  uniform — an  excess  of  stone  in  some 
parts,  excess  of  mortar  in  others ;  and  again  excess  of  sand  in 
places,  and  excess  of  cement  in  others.  This  could  be  doubt¬ 
less  avoided  to  some  extent  by  allowing  the  concrete  to  attain 
some  degree  of  set  before  being  deposited  in  the  water. 
That  concrete  has  been  deposited  under  water  in  many  cases 
is  undoubted,  and  structure  erected  on  it  which  stand  ;  but 
this  does  not  fully  justify  the  practice,  and  it  should  only  be 
resorted  to  in  cases  of  necessity,  and  generally  the  necessity 
can  be  removed  by  a  little  expenditure  of  money. 

219.  There  is  another  manufactured  or  artificial  mortar 
known  as  Pozzuolana,  a  substance  of  volcanic  origin  found  in 
several  countries,  particularly  in  Italy,  also  other  substances 
called  trass  or  terras,  having  nearly  the  same  composition,  and 
composed  mainly  of  silica  and  alumina,  likewise  of  volcanic 
origin.  When  these  substances  are  ground  fine  and  mixed 
with  the  paste  of  rich  lime  they  form  a  substance  possessing 
great  hydraulic  properties,  and  equal  in  strength  to  the  emi¬ 
nently  hydraulic  limes  ;  sand  is  sometimes  added.  The  proper 


104  a  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


proportions  of  these  several  ingredients  would  have  to  be  de¬ 
termined  by  experiment.  These  mortars  were  largely  used  in 
marine  construction  by  the  Romans. 

220.  Artificial  Pozzuolana  is  made  by  burning  clay.  Brick 
or  tile  dust  when  mixed  with  fat  lime  form  a  product  possess¬ 
ing  considerable  hydraulic  energy.  “  Forge  scales,  slags  from 
iron  foundries,  ashes  from  lime-kilns,  containing  cinders,  coal, 
and  lime,  are  artificial  pozzuolanas”  (Gillmore).  Some  mix¬ 
tures  of  these  kinds  seem  to  be  good  substitutes,  when  for  any 
reason  cement  is  difficult  to  secure. 

221.  These  compounds  as  now  made  do  not  seem  to  stand 
the  effects  of  sea-water,  but  some  conflict  and  difference  of 
opinion  exist  on  this  point.  But  our  quick-setting  cements 
made  from  the  natural  cement  stones  and  the  Portland  cements 
can  generally  be  relied  upon  to  resist  the  solvent  action  of  the 
sea-water,  but  they  should  be  allowed  to  get  a  set  before  im¬ 
mersion. 

222.  It  is  a  usual  practice  in  cold  climates  to  suspend  ma¬ 
sonry  work  of  all  kinds  during  the  winter,  as  it  is  a  prevalent 
opinion  that  the  freezing  of  mortar  unfits  it  for  any  ordinary 
purpose;  in  addition,  work  can  never  be  done  as  economically 
in  cold  weather  as  at  other  times.  As  to  the  effects  of  freezing, 
opinions  differ,  it  being  maintained  by  some  that  although  it 
may  retard  the  setting,  it  has  no  ultimate  injurious  effects; 
others  the  contrary.  Lime  mortar,  however,  by  best  authority, 
is  damaged  when  it  alternately  freezes  and  thaws,  but  not 
damaged  when  it  remains  frozen  until  it  has  set,  and  the  same 
may  be  said  of  ordinary  cement  mortars.  Portland  cements 
are  not  affected  even  by  alternately  freezing  and  thawing. 

223.  The  writer  has  been  compelled  on  several  works  of 
importance  to  construct  masonry  nearly  all  the  winter,  and  in 
several  cases  only  stopped  work  when  the  masons  refused  to 
stand  the  exposure  any  longer,  and  in  such  cases  he  anticipated 
the  probability  of  having  to  remove  a  part  of  the  work  in  the 
spring.  The  stones  had  to  be  warmed  and  thawed  out  before 
using;  and  he  has  also  used  mortar  mixed  with  hot  water.  And 
after  the  lapse  of  many  weeks,  through  freezing  and  thawing 


MORTAR. 


105 


weather,  has  found  on  examination  that  little  or  no  damage 
was  done  apparently  to  the  mortar  on  the  top  and  exposed 
joints  of  piers  thus  abandoned  ;  the  mortar  was  powdered  to 
the  depth  of  a  few  inches,  which  being  removed,  the  under¬ 
lying  mortar  was  as  hard  as  could  be  desired  in  the  time,  and 
on  no  occasion  does  he  recall  that  it  was  found  necessary  to 
remove  any  part  of  the  structure.  Mr.  Trautwine  makes  sub¬ 
stantially  the  same  statement  in  his  book.  It  is,  however,  un¬ 
doubtedly  best  to  suspend  work  in  very  cold  weather. 

224.  The  writer  made  a  limited  number  of  experiments, 
when  building  the  bridge  at  Gray’s  Ferry,  Philadelphia,  in  this 
direction  using  several  brands  of  cement.  Briquettes  were 
made  in  the  form  commonly  used  in  the  test  for  tensile 
strength,  of  1  cement,  2  sand,  the  proportion  used  on  the 
work.  One  was  kept  in  the  house  and  occasionally  moistened  ; 
this  did  not  freeze  at  all.  Another  was  frozen,  then  thawed  out, 
and  frozen  again  ;  this  was  repeated  several  times;  and  another 
was  allowed  to  remain  frozen  for  several  days.  These  speci¬ 
mens  were  then  tested  to  destruction  in  a  suitable  machine. 
The  result  was  as  follows  :  The  one  not  frozen  at  all  showed 
the  greatest  strength,  the  one  that  remained  frozen  came  next, 
and  the  one  alternately  frozen  and  thawed  gave  the  least 
strength.  A  sufficient  number  of  experiments  was  not  made 
to  deduce  any  general  law,  or  to  eliminate  those  imperfections 
in  the  samples  that  might  have  existed,  or  to  remove  any 
irregularity  that  might  have  occurred  in  producing  rupture 
when  in  the  testing-machine,  such  as  twisting  or  too  rapid  ap¬ 
plication  of  the  weights,  and  only  gives  this  for  what  it  is 
worth. 

225.  Sometimes  salt  is  mixed  with  the  mortar  to  prevent 
freezing,  but  it  is  a  question  whether  it  will  set  at  all.  It  produces 
a  deliquescing,  sloppy  mass,  inconvenient  to  handle,  and  re¬ 
maining  in  this  state  for  a  long  time  ;  it  is  apt  to  disfigure  the 
face  of  the  masonry,  but  ds-  frequently  used.  This  method 
was  used  at  the  Susquehanna  River  bridge  to  a  considerable 
extent.  The  masonry  was  only  stopped  on  this  work  when  the 
ice  commenced  to  move,  and  boats  could  not  be  held  in  the 


106  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 

river  or  stone  delivered  to  the  piers,  and  no  evidence  has  ex¬ 
isted  of  any  damage  to  the  mortar. 

226.  Pointing-mortar  for  Masonry. — Pointing  masonry  has 
for  its  object  the  protection  of  the  mortar  in  the  joints  ;  to 
effect  this  the  mortar  should  be  cleaned  out  of  the  joints  while 
soft  to  the  depth  of  about  inches,  and  this  should  be  filled 
with  a  specially  prepared  mortar,  and  rammed  as  in  calking ; 
but  in  practice  it  simply  means  shaping  the  joints  so  as  to  pre¬ 
sent  a  neat  appearance,  and  is  often  done  to  disguise  an  irreg¬ 
ular-looking  joint.  The  pointing-mortar  should  be  neat 
cement,  or  at  any  rate  not  more  than  1  sand  to  1  cement,  and 
before  being  applied  should  be  allowed  to  take  a  set,  and 
tempered  with  a  little  water  when  ready  for  use.  Good  point¬ 
ing-mortar  of  1  sand  and  1  Louisville  cement  was  used  at  Point 
Pleasant,  the  mortar  being  mixed  the  night  before  and  allowed 
to  remain  over  night,  and  tempered  with  a  little  water  when 
used. 


Article  XXI. 

SAND. 

227-  SAND  is  essential  in  lime  mortar,  as  lime  paste  shrinks 
and  cracks  on  drying,  but  is  not  in  cement  mortar,  as  cement 
paste  does  not  shrink  or  crack  on  setting,  but  it  is  used  for  the 
sake  of  economy ;  it  also  increases  resistance  to  crushing,  but  it 
diminishes  the  tenacity  of  the  mortar,  the  proportions  varying 
from  1  volume  of  cement  to  3  of  sand,  to  1  of  cement  to  1  of 
sand,  and  in  some  cases  more  sand  is  used,  but  can  hardly  be 
said  to  be  a  good  practice ;  the  common  practice  is  1  of 
cement  to  2  of  sand  by  volume.  Much  has  been  said  and 
written  about  sand,  but  ordinarily  we  have  to  do  the  best  we 
can.  Pit  sand  is  generally  angular,  but  apt  to  be  dirty;  river 
sand,  the  grains  are  apt  to  be  rounded,  and  may  or  may  not 
be  dirty.  As  to  the  size  of  the  grains,  opinions  are  conflicting, 
for  the  best  sand  we  may  say  that  it  should  be  clean  ;  this  is 
generally  determined  by  rubbing  it  in  the  hand  when  damp  :  if 
it  stains  the  hand  it  is  loamy,  and  should  be  avoided.  The 


STABILITY  OF  PIERS. 


IQ/ 


grains  should  be  sharp ;  this  is  determined  by  rolling  the  sand 
in  the  hand  ;  a  well-defined  grating  sound  indicates  sharpness 
of  grain. 

228.  Sand  is  used  with  grains  varying  from  the  size  of  a 
pea  to  a  very  fine  grain,  and  purely  as  a  practical  question  it 
would  seem  to  be  immaterial  what  size  is  used,  provided  the 
grains  are  not  so  large  as  to  cause  the  stones  to  ride  upon 
them,  and  to  avoid  this  danger  the  sand  is  generally  required 
to  be  screened.  The  size  of  the  meshes  of  the  screen  depend 
upon  the  purpose  for  which  the  sand  is  used,  but  are  commonly 
not  over  one  eighth  of  an  inch  square ;  this  will  not  pass  a 
grain  much  over  one  sixteenth  of  an  inch  square,  as  the 
screening  is  ordinarily  done  in  practice,  which  would  not  be  ob¬ 
jected  to  for  any  kind  of  work ;  this  would,  however,  be  called 
a  coarse  sand.  Other  sands  vary  even  to  almost  imperceptible 
powder  ;  but  if  it  is  really  clean  sand  the  grating  sound  can 
still  be  detected,  and  the  distinct  grains  easily  seen  by  an 
ordinary  magnifying-glass ;  but  this  very  fine  sand  is  also  ob¬ 
jected  to,  and  a  medium  grain  seems  to  give  more  satisfaction. 
Damp  sand  if  clean  when  pressed  in  the  hand  will  not  hold  its 
shape  on  opening  the  hand,  but  will  split  and  fall  away ;  this  is 
probably  the  best  test  as  to  the  cleanness  of  sand,  as  the  pres¬ 
ence  of  clay  or  loam  would  cement  the  grains  together.  It  is 
claimed  that  the  finer  the  sand  the  more  cement  is  required  to 
make  the  mortar.  Sand  from  salt  water  is  objected  to  by 
many,  unless  it  is  well  washed  before  using.  Sand  is  generally 
composed  of  different-sized  grains,  which  is  a  favorable  condi¬ 
tion  for  economy. 

Article  XXII. 

STABILITY  OF  PIERS. 

229.  PIERS  can  give  way  by  sliding  along  some  horizontal 
bed-joint,  or  by  overturning  around  some  edge  of  the  masonry, 
or  by  crushing  the  material  of  which  it  is  made. 

230.  The  pressures  tending  to  cause  sliding  are  the  force  of 
the  wind,  the  force  of  the  current  acting  directly  on  the  end  of 


Io8  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


the  pier  and  crib,  or  acting  on  a  mass  of  ice  or  driftwood, 
which  sometimes  accumulates  above  the  pier  for  a  greater  or 
less  distance  on  either  side,  and  often  extending  from  pier  to 
pier.  In  such  cases  the  ice  or  drift  sinks  and  collects  in  very 
large  masses,  presenting  a  large  exposed  surface  to  the  current ; 
the  effect  of  this  is  to  cause  the  current  to  be  much  more 
rapid  underneath  the  compact  mass,  endangering  the  destruc¬ 
tion  of  the  pier  by  undermining  or  scouring,  and  at  the  same 
time  largely  increasing  the  pressure  on  the  piers. 

231.  The  writer  has  observed  closely  and  anxiously  the 
action  of  a  large  mass  of  drift  and  ice  under  varying  condi¬ 
tions,  and  a  brief  description  of  these  cases,  differing  entirely, 
may  not  be  uninteresting,  and  in  some  degree  instructive, 
though  he  is  unable  to  add  anything  in  the  way  of  a  formula 
or  law  at  all  practical  or  useful.  The  action  may  be  due  to 
the  ice  and  drift  while  stationary,  or  moving  as  a  whole  or  in 
detached  masses. 

232.  First,  while  Stationary. — Expansion  and  Contraction  of 
Ice. — Ice  on  the  surface  of  a  lake  may  exert  an  enormous 
force,  sufficient  to  move  heavy  masonry  piers,  caused  by  alter¬ 
nate  expansion  and  contraction  under  changing  temperature ; 
even  if  the  pier  is  protected  by  a  sloping  surface  at  the  water’s 
edge  ;  the  adhesion  of  the  ice  to  the  stone  may  be  so  great  that 
it  will  exert  against  the  pier  a  thrust  due  to  its  full  crushing 
strength  before  it  will  fracture.  The  crushing  strength  of  ice  as 
given  by  different  authorities  varies  very  greatly,  depending  on 
its  thickness,  purity  of  water  from  which  it  is  formed,  and  also 
with  its  temperature,  the  limits  being  from  400  to  IOOO  lbs. 
per  square  inch.  This  enormous  pressure,  acting  over  a  con¬ 
siderable  surface  and  often  with  a  long  lever  arm,  may  exert  a 
very  great  overturning  moment.  It  is  stated  in  Engineering 
News,  Jan.  12,  1893,  that  a  pier  weighing  1000  tons  was  not 
only  lifted,  but  held  up  under  passing  trains  ;  and  piers  built 
on  pile  foundations  were  thrown  out  of  line  from  2  to  12 
inches,  the  ice  being  from  10  to  12  inches  thick.  When  the 
ice  was  cut  away  the  piers  moved  back  nearly  to  their  original 
positions.  This  effect  was  attributed  solely  to  expansion  of 


STABILITY  OF  PIERS. 


IOg 


the  ice  sheet.  The  writer’s  observations  during  a  very  severe 
winter  at  Havre  de  Grace,  on  the  action  of  ice  formed  on  the 
Susquehanna  River  against  piers,  does  not  correspond  with  the 
above  estimate  of  the  force  of  adhesion,  and  he  does  not 
understand  how  the  ice  can  adhere  to  the  pier  at  all  unless 
the  water  is  perfectly  still  ;  any  oscillation  whatever  of  the  sur¬ 
face  causing  alternate  rising  and  falling  of  the  ice-sheet  grinds 
and  breaks  the  ice  in  contact  with  the  surface  of  the  pier,  con¬ 
sequently  preventing  time  for  any  adhesion  to  exist.  This  was 
noticeable  on  four  or  five  piers  at  the  Susquehanna  Bridge  ;  the 
ice  was  some  15  inches  thick,  and  remaining  for  months,  with¬ 
out  movement  in  a  horizontal  direction,  during  great  changes  of 
temperature.  It  was  unsafe  to  approach  the  piers  too  closely, 
that  is,  within  a  foot  or  two.  The  writer  does  not  question 
the  great  pressure  that  would  be  exerted  by  the  ice  on  the 
pier  while  expanding,  except  in  so  far  as  this  condition  imme¬ 
diately  surrounding  the  piers  would  affect  the  pressure  on  the 
piers  from  expansion,  as  this  effect  can  only  appreciably  exist 
when  the  ice  sheet  has  a  very  great  expanse.  The  sheet  at 
the  Susquehanna  extended  between  two  and  three  miles  in  the 
direction  of  the  longer  horizontal  axis  of  the  pier,  but  only,  of 
course,  a  short  distance,  about  500  feet,  in  the  direction  of  the 
shorter  axis.  Even,  however,  assuming  a  strong  adhesion  of 
the  ice  to  the  masonry  of  the  pier,  a  wide  range  of  temperature, 
a  great  expanse  of  ice  in  one  or  more  directions,  the  danger 
arising  from  expanding  ice  can  be  economically  avoided  by 
cutting  narrow  channels  through  the  ice  parallel  to  the  axis  of 
the  bridge. 

233.  Secondly ,  while  Moving  as  a  Whole  or  in  Detached 
Masses. —  The  Breaking  and  Fiozving  of  the  Ice  at  Gray  s  Ferry , 
Philadelphia,  in  the  Schuylkill  River  and  the  Ohio  River  at  sev¬ 
eral  points,  and  the  Drift  Gorge  on  the  Tombigbee  River,  Ala¬ 
bama. — In  the  Susquehanna  at  Havre  de  Grace  there  is  a 
broad  stretch  of  very  deep  water  divided  into  two  channels  by 
Watson’s  Island,  extending  about  2^  miles  to  a  point  above  at 
Port  Deposit,  where  rocky  ledges  seem  to  rise  abruptly,  a 
large  part  of  which  is  exposed  at  low  water  ;  this  continues  for 


HO  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 

miles  above,  constituting  the  falls  or  rapids.  Below  these  the 
current  is  comparatively  slow,  hardly  perceptible  in  low  water ; 
this  water  freezes  to  the  depth  of  two  or  more  feet,  and  the 
ice  rising  and  falling  with  the  tide,  is  generally  broken  for 
several  feet  along  the  shores  or  around  obstructions,  such  as 
piers,  thereby  free  to  move  in  a  body.  When  the  ice  breaks 
above  in  the  spring  rises,  it  is  brought  down  by  the  rapid  cur¬ 
rent,  and  coming  in  contact  with  the  solid  mass  of  ice  at  the 
end  of  the  rapids,  it  is  thrown  up  and  down  and  sidewise, 
flooding  the  streets  of  Port  Deposit  with  water  and  masses  of 
broken  ice  on  one  side,  and  the  tow-path  of  the  canal  on  the 
other  side,  of  the  river.  Under  this  immense  power  the  entire 
mass  of  ice,  three  or  four  miles  long  and  more  than  half  a  mile 
wide,  moves  as  a  whole, — not  more  than  8  or  iofeet,  probably, 
— crushing  everything,  except  masonry,  in  its  movement ;  this 
seems  to  relieve  the  pressure  to  a  great  extent,  but  the  rapid 
flow  of  water  under  the  ice  can  easily  be  observed.  The  ice 
will  then  remain  in  this  position  for  some  time. 

234.  It  will  have,  however,  crushed  or  broken  or  torn  up 
any  temporary  breakwater  made  of  large  numbers  of  piles,  and 
crushed  into  splinters  the  timber  of  the  coffer-dams,  breaking 
and  twisting  strong  iron  bolts  and  rods  two  inches  in  diam¬ 
eter  ;  where  it  has  struck  the  cutwaters  of  masonry  piers,  it 
will  be  split  from  50  to  100  feet  above  the  pier,  the  ice  mount¬ 
ing  the  pier  in  great  masses.  Evidently  the  broken  ice  again 
begins  to  be  accumulated  at  the  upper  end  above  and  below 
the  broken  sheet  of  ice,  and  commonly  believed  to  reach  to  the 
bed  of  the  river ;  but  this  can  hardly  be,  as  in  this  case  the 
water  would  flow  for  some  distance  over  the  top  of  the  ice  be¬ 
fore  such  an  immense  sheet  could  adjust  itself  to  the  new  con¬ 
ditions,  which  does  not  occur.  At  this  time  another  movement 
takes  place,  with  similar  results  as  before  ;  and  this  may  continue 
for  some  time,  alternately  moving  and  stopping,  the  main  sheet 
of  ice  still  remaining  solid,  and  in  the  writer’s  observation  only 
breaking  up  when  the  warm  weather  has  simply  rotted  it.  He 
believes,  therefore,  that  the  moving  force  is  the  action  of  the 
current  upon  the  large  face  at  the  upper  end  of  packed  ice  both 


STABILITY  OF  PIERS. 


Ill 


above  and  below  the  main  sheet,  and  that  it  is  a  mere  ques¬ 
tion  of  the  ice  or  the  pier  giving  way.  A  square-ended  pier 
would  under  such  circumstances  be  put  to  a  severe  trial,  but  a 
good  cutwater  ploughs  through  the  ice  with  hardly  a  tremor, 
aided  as  it  is  by  the  ice  rising  on  the  sloping  cutwater  and 
splitting  in  almost  a  straight  line. 

235.  The  moment  of  this  force  cannot  be  accurately  or  ap¬ 
proximately  estimated  ;  the  strongest  kinds  of  timber  frames, 
faced  with  iron  rails,  are  simply  crushed  and  splintered  into 
kindling-wood. 

236.  Mr.  Weisbach  and  others  give  formulae  for  the  amount 
of  force  exerted  ;  but  the  elements  of  it  are  unknown,  and  all 
that  can  be  done  is  to  assume  a  certain  value  for  the  area  of 
surface  pressed,  velocity  of  current,  and  some  unknown  factor 
depending  on  the  shape  of  the  pier,  and  obtaining  a  result 
which  could  have  no  practical  value. 

237.  By  observing  the  actual  result  and  determining  the  re¬ 
sistance  to  splitting  of  the  ice  for  a  distance  above  the  pier,  we 
will  have  part  of  the  resistance  to  the  moving  force.  Much  of 
it,  however,  is  taken  up  by  friction  of  the  ice  as  it  moves  along 
the  shore  lines,  wider  at  some  and  narrower  at  other  parts.  The 
writer  has  therefore  simply  determined  the  extent  of  the  force 
on  the  pier  by  multiplying  the  area  of  the  split  surface  by  10 
tons  to  the  square  foot,  this  figure  being  simply  assumed  (it 
may  be  very  far  from  the  correct  value,  as  so  much  depends 
upon  the  temperature  and  the  condition  of  freezing;  from  some 
actual  experiments  the  tensile  strength  has  been  found  to  vary 
from  140  to  200  pounds  per  square  inch),  in  order  to  illustrate 
the  manner  of  arriving  at  the  actual  force  exerted. 

238.  The  same  may  be  said  of  accumulated  driftwood,  ex¬ 
cept  that  there  is  no  means  of  arriving  at  the  force  necessary  to 
break  through  a  large  mass  of  drift.  This  generally  occurs  in 
those  rivers  in  which  the  water  gets  out  of  its  banks  and  spreads 
over  the  country,  which  is  the  case  in  almost  all  southern  rivers, 
many  of  which  rise  rapidly  in  one,  two,  or  three  days  to  the 
height  of  40  to  50  feet,  collecting,  from  miles  on  either  side  of 
the  river,  in  places,  immense  amounts  of  drift,  entire  trees, 


I  12 


A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


logs,  brushwood,  etc.,  which  form,  on  meeting  an  obstruction, 
a  mattress  dam  from  pier  to  pier,  and,  to  a  depth  of  io  to  12 
feet,  with  logs  and  drift  at  all  conceivable  angles,  extending 
from  bank  to  bank  and  covering  acres  of  water  above. 

239.  In  the  Tombigbee  River  this  was  observed  in  a  thirty- 
five-foot  rise,  but  a  timber  coffer-dam  (around  a  pivot  pier 
built  of  brick),  of  verticals  and  two  courses  of  3-inch  plank  held 
down  to  the  crib  by  2-inch  iron  rods,  octagonal  in  plane,  well 
braced  on  the  inside,  resisted  this  pressure  without  springing  a 
leak  of  any  consequence.  This  could  not  have  stood  an  ice  move¬ 
ment  such  as  above  described,  though  apparently  it  looked 
equally  as  formidable,  and  would  not  yield.  All  that  could  be 
done  was  to  keep  a  number  of  men  with  iron-pointed  poles 
standing  on  the  lower  end  of  the  drift  gradually  working  piece 
after  piece  from  the  mass,  and  steam-tugs  pulling  with  hawsers 
at  the  larger  and  longer  pieces;  and  occasionally  under  this 
action  it  would  be  so  far  disentangled  that  immense  masses 
would  be  carried  by  the  current  between  the  piers. 

240.  At  the  Schuylkill,  after  a  very  cold  winter,  the  river 
was  frozen  solidly  over,  but  owing  to  the  number  of  bridges 
above,  and  the  sinuosities  of  the  stream,  there  was  little  chance 
for  a  similar  movement  of  the  ice  to  that  at  the  Susquehanna, 
and  in  the  break-up  in  the  spring  rise  it  was  simply  a  rapid 
flow  of  immense  masses  of  broken  ice.  This  did  not  gorge  to 
any  extent ;  and  though  it  broke  barges  loose  strongly  anchored 
and  secured,  upsetting  some,  carrying  others  off,  and  this  flow 
continuing  for  several  days  acting  on  a  square-end  coffer-dam 
bolted  to  cribs,  the  coffer-dam  being  20  or  more  feet  wide  and 
12  to  15  feet  high,  with  no  masonry  built  inside,  but  good 
strong  timber  bracing  being  in  place,  the  water  rising  nearly 
to  the  top  of  the  dam,  no  damage  was  caused  to  the  dam  at 
all,  did  not  fill  with  water,  and  work  proceeded  at  once. 

241.  This  dam  had  no  protection  of  any  kind  above  it, 
whereas  the  Susquehanna  dam  of  the  same  construction,  though 
double  the  size,  was  protected  by  a  number  of  piles,  numbering 
75  or  100,  driven  close  together  in  the  form  of  a  breakwater, 
which  was  crushed  to  pieces,  and  we  never  saw  or  heard  of  the 


WA  TER  -WAY  IN  CUL  VER  TS. 


1 13 

piles  afterwards.  These  piles  were  not  braced  rigidly  together, 
as  it  was  vainly  thought  better  to  allow  the  piles  to  gradually 
yield  to  the  pressure,  and  thereby  break  the  force  of  the  press¬ 
ure  to  some  extent ;  but  to  no  purpose. 

242.  In  the  one  case  the  movement  was  slow  and  powerful ; 
in  the  second  no  movement  of  the  drift  as  a  whole  observable, 
notwithstanding  the  very  great  velocity  of  flow  of  water;  and 
in  the  third  large  masses  of  broken  ice  moving  with  a  very 
great  velocity,  their  comparative  destructive  effects  being  as 
above  described. 

243.  The  action  of  the  wind  upon  the  piers  has  been 
explained  and  illustrated  by  an  example.  Experiment  proves 
that  its  force,  either  alone  or  combined  with  the  one  above 
discussed,  will  neither  cause  ordinary  piers  to  slide  nor  over¬ 
turn.  The  piers  of  the  P.,  W.  &  B.  bridge  at  Havre  de  Grace 
are  apparently  unusually  light,  only  extending  a  few  feet  above 
high-water,  and  carrying  very  short  spans,  giving  what  might 
be  called  the  minimum  elements  of  resistance  to  these  press¬ 
ures.  These  certainly  seem  safe  against  such  pressures.  It 
is  true  that  they  are  protected  by  the  island  just  above  to  a 
considerable  extent. 

244.  As  to  the  crushing  resistance  of  the  masonry  under 
the  influence  of  the  weight  of  the  pier,  the  wind,  and  the  ice, 
there  is  such  a  wide  margin  of  safety  in  the  strength  of  the 
stone  that  no  apprehension  of  this  kind  need  be  felt,  as  in 
practice  the  greatest  pressure  that  can  possibly  occur  would 
not  exceed  the  mean  pressure  more  than  about  two  times, 
which  would  give  a  large  factor-of-safety. 

Article  XXIII. 

WATER-WAY  IN  CULVERTS. 

245.  In  determining  the  necessary  water-way  for  box  and 
arch  culverts,  so  many  and  so  varying  conditions  arise  that 
it  is  impossible  to  deduce  even  an  approximate  formula  that 
would  be  applicable  to  even  a  single  stream,  and  on  a  hundred 
miles  of  road  every  ditch  and  every  stream  would  require  a 


1 14  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 

different  coefficient,  the  value  of  which  could  only  be  wildly 
guessed  at  without  a  survey,  the  expense  of  which  would  be 
many  times  more  than  the  cost  of  a  culvert  much  larger  than 
actually  necessary.  The  more  common  formulae  are  simply 
based  on  the  supposition  that  the  area  of  the  water-way  in 
square  feet  is  equal  to  some  root  of  the  drainage  area  in 
acres  multiplied  by  a  constant  the  value  of  which  is  unknown; 
for  instance,  area  of  water-way  in  square  feet 

=  C  V drainage  area  in  acres 

(Myers’  formula). 

246.  Practically  no  formula  is  generally  used.  The  engineer 
can  generally  find  in  the  neighborhood  some  highway  bridge 
which  by  observation  or  information  obtained  from  residents 
will  serve  as  a  guide ;  or  in  ,the  absence  of  this,  places  can  be 
found  in  which  the  water  is  confined  between  the  banks  of  the 
stream  in  times  of  the  highest  flood  known  in  the  locality,  or 
by  taking  cross  sections  of  the  stream  as  indicated  by  the  limits 
of  high-water,  but  here  the  velocity  of  discharge  would  be  un¬ 
known.  Altogether  we  have  no  data  upon  which  to  base  in 
the  beginning  an  intelligent  opinion,  but  in  general,  owing  to 
the  rapidity  of  railway  construction  now,  temporary  openings 
are  left  in  the  way  of  trestles,  which  can  always  be  made  suffi¬ 
ciently  long  to  give  full  water-way,  leaving  the  question  of  per¬ 
manent  culverts  to  be  settled  later. 

247.  Good  judgment  generally  determines  the  proper  size 
of  culverts.  Sometimes  a  single  opening  of  a  foot  square  built 
of  rough  stone  will  carry  the  necessary  water,  throwing,  over 
and  around  this,  broken  stone  (before  dumping  the  ordinary 
earth),  which  would  allow  any  unusual  discharge  to  find  its  way 
through  without  any  damage  to  the  embankment,  or  small 
terra-cotta  pipes  from  6  inches  to  1  foot  in  diameter  may  be 
used.  These  would  be  applicable  simply  at  depressions  where 
there  are  no  permanent  or  well-defined  ditches  or  streams,  but 
not  where  the  embankment  would  dam  up  the  water  in  case  of 
hard  rains,  which  might  by  seeping  through  the  earth  cause 
settlement  and  possibly  danger. 


ARCH  CULVERTS. 


115 

248.  For  any  well-defined  ditch  or  stream  a  culvert  2  feet 
square,  increasing  to  3  or  4  feet  wide  by  5  feet  high,  as  the  cir¬ 
cumstances  seem  to  require,  will  answer  every  purpose.  If  a 
larger  water-way  is  required  use  the  double  box  of  correspond¬ 
ing  dimensions,  and  if  this  is  not  sufficient  the  arch  culvert 
should  be  used.  The  side  walls  vary  in  thickness  from  2  to  3 
feet  according  to  height,  and  covering  stones  from  10  to  16 
inches  thick,  according  to  the  length  of  the  span. 

249.  The  vitrified  terra-cotta  pipes  used  are  what  are  known 
as  double-strength  glazed  pipe.  They  are  now  comparatively 
cheap,  costing  at  most  from  twenty  cents  to  one  dollar  per 
foot,  according  to  size,  and  easy  to  handle  ;  but  there  is  always 
more  or  less  danger  of  cracking  or  breaking,  and  for  this  reason, 
if  no  other,  masonry  culverts  are  preferred.  These  cost  from 
two  to  five  dollars  per  foot  of  length,  depending  upon  conveni¬ 
ence  of  material  and  kind  of  work  required. 

250.  Cast-iron  pipes  have  been  used  extensively  in  some 
sections  of  the  country  in  diameters  as  great  as  4  feet  and  in 
lengths  that  may  be  required.  These  cost  from  one  to  eight 
dollars  per  lineal  foot,  according  to  their  weight.  Special  care 
is  required  in  laying  these  pipes  to  prevent  undue  settling. 

Article  XXIV. 

ARCH  CULVERTS. 

251.  WHEN  a  larger  water-way  is  required  than  the  limit  of 
box  culverts,  say  4  feet  span,  arch  culverts  are  constructed. 
These  differ  only  in  the  size  from  ordinary  arches,  which  have 
been  fully  discussed,  except  that  the  spaces  between  the  abut¬ 
ments  are  generally  paved,  and  an  apron  wall  is  built  from  side 
to  side  at  the  ends  of  the  arch  of  a  depth  somewhat  greater 
than  the  foundation  bed  of  the  side  walls  or  abutments,  the 
paving  also  extending  above  and  below  between  the  wing  walls. 

252.  The  usual  manner  of  connecting  the  wing  walls  with 
the  head  walls  is  to  throw  them  well  back  from  the  arch  ring 
and  then  build  them  to  the  proper  height  with  the  usual  batter, 


Il6  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 

and  thickness  proportioned  as  in  retaining-walls,  the  angle 
of  the  wing  being  determined  by  circumstances — generally 
not  more  than  30  degrees  with  the  axis  of  the  arch.  The  only 
objection  to  these  are  the  square  shoulders  presented  at  the 
entrance  of  the  arch,  forming  a  lodgment  for  ice  and  drift. 
This  can  be  avoided  to  some  extent  by  starting  the  wing  wall 
at  the  front  of  the  abutment,  and  carrying  it  up  vertically  to  the 
springing  line,  and  then  commence  the  batter  wall  either  at  the 
springing  line  of  the  soffit,  or  leaving  an  offset  at  the  springing 
line  equal  to  the  thickness  of  the  arch  ring  and  commencing 
the  batter  wall  at  the  back  or  extrados  of  the  arch  ring.  This 
was  the  plan  adopted  in  the  Philadelphia  extension  of  the  B. 
&  O.  Ry.,  except  that  the  wing  wall  to  the  height  of  the  spring¬ 
ing  line  was  carried  up  on  a  warped  or  twisted  surface,  that  is, 
vertical  where  it  adjoined  the  abutment,  but  assuming  a  gradu¬ 
ally  increasing  batter  to  the  end  of  the  wing.  This  presents 
no  shoulder  to  the  height  of  the  springing  line,  but  presents 
shoulders  above  that  line.  It  allows  the  wing  wall  to  be  bonded 
for  its  entire  height  into  the  head  wall. 

253.  For  any  very  small  arch  from  5  to  15  feet  span  they  are 
generally  full-centre  circular  arches.  For  spans  from  20  to  30 
feet  the  segmental  arches  offer  some  advantages  :  for  the  same 
length  of  intrados  give  a  little  longer  span  and  the  area  of  the 
water-way  is  a  little  greater,  and  for  the  same  length  of  span 
there  is  a  little  less  masonry,  and  does  not  require  so  great  a 
rise,  which  is  often  an  important  consideration.  On  the  con¬ 
trary,  the  segmental  arch  requires  greater  thickness  of  arch  ring 
and  abutments. 

254.  Sometimes  the  wing  walls  are  perpendicular  to  the 
axis  of  the  arch,  or  in  other  words  a  simple  extension  of  the 
head  walls  in  a  straight  line  ;  these  offer  no  advantages,  and 
are  not  used  in  any  but  very  small  arches. 

255.  Formula  for  the  thickness  of  the  arch  ring  have  already 
been  given.  An  arch  ring  somewhat  thicker,  both  for  circular 
and  segmental  arches,  is  advisable;  the  cost  will  not  be  much, 
if  any,  more,  and  presents  a  better  and  stronger  appearance. 

256.  Trautwine  gives  the  following  rule  for  determining 


ARCH  CULVERTS. 


IV/ 


the  thickness  of  abutments  for  arches  in  feet  at  the  springing 
line,  for  any  abutment  the  height  of  which  does  not  exceed 
times  the  thickness  at  the  base.  Thickness  of  abutment 
at  springing  equal  to 


radius  in  feet 

_ 5 


rise  in  feet  .  t  ^ 

- b  2  feet 

io 


The  radius  in  this  formula  is  that  of  a  circle  passing  through 
the  two  springing  lines  and  the  crown,  on  the  soffit.  As  it  is 
always  practicable  to  find  a  circumference  passing  through 
any  three  points  in  the  same  plane,  this  formula  is  applicable 
to  a  semicircular,  a  segmental,  or  elliptical  arch.  Laying  off 
this  distance  and  the  height  of  the  wall  perpendicular  to  it, 
and  at  the  bottom  of  this  line  drawing  another  line  from  one 
half  to  two  thirds  of  the  height,  the  line  closing  this  quadrilat¬ 
eral  would  represent  the  surface  of  the  back  of  the  wall.  This 
thickness  is  given  to  resist  the  thrust  on  the  wall  arising  from 
the  earth  pressure  of  the  embankment  of  any  height  over  and 
around  the  wall.  This  does  not  take  into  consideration  the 
thrust  on  the  abutment  by  the  arch,  which  is  in  the  opposite 
direction  to  the  earth  pressure  and  at  least  neutralizes  it  in 
part,  nor  any  support  arising  from  the  wing  walls.  This  will 
no  doubt  give  sufficient  thickness  in  any  case,  but  to  resist  the 
effects  of  the  rolling  load  it  is  supposed  that  the  earth  has  been 
deposited  behind  the  abutments. 

257.  Applying  this  formula  to  spans  6',  10',  l6',  with  a  rise 
of  one  sixth  of  the  span,  segmental  arch,  in  the  first  case  the 
radius  equal  to  5,  in  the  second  equal  to  8.2,  and  in  the  third 
equal  to  13.2. 

The  rise  in  feet  would  be,  respectively,  =  1,  in  the  second 
—  1.7,  and  in  the  third  =  2.7.  Thickness  (Trautwine’s  for¬ 
mula)  of  abutment  at  top  in  feet  =  3.1,  in  the  second  =  3.8, 
and  in  the  third  =  4.92.  Actual  thickness  of  abutment  at  top 
in  feet  in  same  cases  —  3.3,  in  the  second  =  4.6,  and  in  the 
third  =  5.6. 

Actual  depth  of  key-stone  at  crown  =  1.0  foot,  in  the  second 


1 1 8  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 

—  1.3  feet,  and  in  the  third  =  1.6  feet.  For  semicircular 
spans,  as  above,  actual  depth  of  key-stone  at  the  crown  =  0.60 
foot,  in  the  second  =  1.0  foot,  and  in  the  third  =  1.2  feet. 
Actual  thickness  of  abutment  at  top  =  3.0  feet,  in  the  second 
=  3.6  feet,  and  in  the  third  =  4.6  feet. 

Trautwine’s  results  agree  very  closely  with  those  in  actual 
use  for  a  semicircular  or  full  centre  arch,  but  smaller  than  the 
practice  for  segmental  or  elliptical  arches.  The  practical  rule 
is  to  add  one  third  part  additional  in  these  cases,  but  this 
would  be  considerably  in  excess  of  the  actual  thickness  of 
abutments  given  in  the  table  above. 

258.  But,  as  stated  before,  the  best  rule  is  to  extend  the 
line  of  pressure  from  the  top  to  the  bottom  of  the  abutment,, 
and  if  this  line  remains  in  the  middle  third  of  its  thickness,  it 
will  be  stable  so  far  as  the  thrust  of  the  arch  is  concerned,  and 
determine  its  stability  against  the  earth  pressure  by  the  em¬ 
pirical  rules  for  the  thickness  of  retaining-walls ;  the  least 
thickness  thus  determined  will  certainly  be  ample,  as  the  two 
pressures  balance  each  other  in  part. 

259.  But  the  earth  pressure  is  hard  even  to  approximate,, 
as  evidently  the  condition  would  be  that  of  a  surcharged  wall ; 
for  by  removing  the  arch  ring  and  the  earth  above  it  the 
abutment  would  simply  be  a  foot  wall  at  the  bottom  of  a  mass 
of  earth  sloping  away  from  the  top  of  the  wall  at  the  angle  of 
repose,  either  resulting  from  the  removal  of  the  earth  to  that 
slope  or  allowing  it  to  assume  its  natural  slope.  At  the  solu¬ 
tion  of  this  the  wonderful  penetration  of  Mr.  Rankine  seems 
to  falter,  but  he  suggests  the  following: 

t"  i/  w'  x  ,  .  ,  . 

—  —  cos  (p  V  g —  X  7-3 — • — t  +  2c  ~  (x  -f-  2c), 
x  0 qw  1  -j-  sin  0  '  7 

in  which  t"  —  the  thickness  of  the  surcharged  wall,  c  =  height 
of  surcharge,  x  =  height  of  wall,  0  =  angle  of  repose  of  ma¬ 
terial,  w'  —  weight  of  a  cubic  foot  of  the  earth,  w  —  weight  of 
a  cubic  foot  of  the  masonry  in  the  wall.  Mr.  Trautwine  gives 
the  formula  as  follows:  True  theoretical  thickness  =  weight  of 
earth  x  0.643,  in  which  the  weight  of  the  earth  is  that  of  a 


ARCH  CULVERTS. 


U9 

triangular  prism,  whose  base  is  formed  by  the  height  of  the 
wall,  the  prolongation  of  the  plane  of  maximum  pressure  to  its 
intersection  with  the  slope  of  the  ground,  thence  by  the  length 
of  this  slope  to  the  top  of  the  wall.  The  moment  of  this 
equated  to  the  moment  of  the  weight  of  the  wall  will  enable 
us  to  determine  the  thickness  as  in  retaining-walls. 

260.  With  a  surcharge  two  times  the  height  of  the  wall,  he 
gives  the  following  as  safe  thickness:  cut  stone  0.58,  rubble  or 
brick  0.63,  of  the  height  of  the  wall;  and  for  a  surcharge  nir.e 
times  the  height  of  the  wall,  cut  stone  0.65,  rubble  or  brick 
0.80,  of  the  height  of  the  wall.  These  thicknesses  are  roughly 
one  and  one  half  times  of  those  when  the  earth  is  level  with 
the  top  of  the  wall. 

261.  Whether  these  are  right  or  wrong,  they  have  but 
little  bearing  in  the  case  of  the  abutments  of  the  arch  ;  for  if 
the  earth,  as  is  often  the  case  in  railroad  abutments,  and 
which  is  the  general  case  for  culverts  under  highway  bridges, 
only  extends  a  few  feet  above  the  top  of  the  arch,  it  evidently 
would  be  considered  safe  to  consider  the  half  arch,  with  its 
abutment  and  weight  above,  as  the  equivalent  of  a  vertical¬ 
faced  wall  of  the  height  of  the  embankment,  and  find  the 
thickness  of  the  wall  to  insure  stability,  as  in  retaining-walls, 
or  using  the  thickness  followed  in  practice,  that  is,  from  two 
fifths  to  one  half  the  height.  Take,  for  example,  a  semi¬ 
circular  arch  of  20  feet  span,  the  rise  10  feet  -f-  arch  ring 
-f-  spandrel  =  say  to  15  feet,  and  height  of  abutment  between 
footing-courses  and  springing  line  10  feet,  the  equivalent  verti¬ 
cal-faced  wall  would  then  be  25  feet,  and  two  fifths  of  this  would 
be  10  feet  =  required  thickness,  or  one  half  the  height  would  be 
1 2%  feet.  This  would  evidently  be  ample.  Mr.  Trautwine’s 
rule  for  the  thickness  of  abutments  would  give  5  feet  at  top 
and  6.6  at  base. 

262.  The  writer  does  not  believe  that  any  greater  height  of 
embankment,  no  matter  how  high,  would  require  any  greater 
thickness  of  abutment  wall,  and  even  doubts  if  the  pressure 
would  be  any  greater ;  but  if  it  is,  the  greater  stability  of  the 
wall,  resulting  from  increase  of  weight  of  material  above, 


120  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 

would  at  least  balance  the  increased  thrust.  The  abutment 
would  only  give  way  by  sliding  or  crushing,  as  the  arch  as  a 
whole  would  be  stable  against  overturning  on  account  of  the 
balanced  pressure  from  the  two  sides. 

263.  It  is  not  uncommon  to  see  little  foot  walls  supporting 
as  it  were  sloping  embankments  ten  times  their  own  height, 
even  when  the  material  would  continue  to  cave  and  slide 
down.  It  is  true  that  these  walls  are  always  made  of  thick¬ 
nesses  equal  to  their  heights,  or  even  greater. 

264.  In  these  calculations  neither  the  adhesion  of  the  mate¬ 
rial,  nor  the  support  that  the  earth  gives  to  itself  by  a  tendency 
to  be  self-supporting  by  arching  itself  above  the  arch,  nor  the 
adhesion  or  cohesion  of  the  mortar  in  the  masonry,  is  con¬ 
sidered. 

265.  If  the  above  reasoning  is  not  true,  the  linings  of  tun¬ 
nels  would  be  ridiculously  thin,  being  only  a  few  bricks  in  thick¬ 
ness,  say  from  two  to  three  feet,  or  from  5  to  8  rings  of  brick 
laid  flatwise.  It  is  true  that  such  linings  would  not  hold  the 
pressure  if  it  should  once  take  a  move,  but  are  ample  to  prevent 
any  movement.  Conditions  are  of  course  different  under  a 
bank  of  loose  earth  thrown  up,  even  after  the  lapse  of  time. 

Article  XXV. 

THE  COST  OF  WORK. 

266.  It  is  considered  by  some  useless  to  give  the  cost  of 
any  particular  kind  of  work,  as  this  depends  upon  so  many  con¬ 
tingencies  and  these  varying  rapidly  from  year  to  year,  depend¬ 
ing  upon  the  abundance  or  scarcity  of  materials,  the  amount  of 
work  being  carried  on,  the  character  of  the  specifications,  the 
scarcity  or  abundance  of  labor,  the  time  in  which  the  work  is 
to  be  completed,  as  well  as  .  the  amount  of  work  to  be  done 
at  any  one  time  and  place :  all  of  these  things  render  anything 
but  an  approximation  to  the  cost  uncertain.  It,  however, 
serves  as  a  guide.  It  can  be  stated  as  a  general  principle,  that 
it  will  generally  prove  economical  in  the  end  to  pay  fair  prices 
to  responsible  parties,  and  not  to  endeavor  to  get  work  done 
at  a  price  less  than  it  is  really  worth,  and  thereby  secure  only 


THE  COST  OF  WORK. 


1 2 1 


Irresponsible  contractors,  who  will  not  only  execute  the  work 
badly,  will  never  execute  it  in  the  specified  time,  will  give  all 
kinds  of  trouble,  will  endeavor  to  impose  on  you  by  numerous 
extra  bills,  by  enormous  charges  for  extra  work,  but  too 
often,  after  performing  that  part  of  the  work  in  which  the 
greatest  profit  exists,  abandon  the  work,  leaving  large  unpaid 
bills  for  material  and  labor  to  be  paid  by  the  company,  and  a 
larger  cost  to  complete  the  remaining  work,  than  would  have 
been  really  required  to  do  the  entire  work,  if  let  to  capable 
and  responsible  men.  Almost  all  important  work  is  done  by 
contract,  because  it  is  the  cheaper  in  the  long-run. 

267.  Ordinary  brick  or  stone  masonry,  concrete  and  earth¬ 
work,  are  generally  paid  for  by  the  cubic  yard.  Brickwork  for 
the  walls  of  houses  either  by  the  cubic  yard  or  by  the  perch  or 
face  measure,  for  walls  of  definite  thickness,  say  one  and  one 
half  brick  or  12  inches  ;  if  the  wall  is  two  times  one  and  one  half 
bricks,  the  area  of  the  surface  is  doubled  ;  if  only  one  brick  thick, 
then  two  thirds  of  the  surface  is  taken.  All  openings  are  some¬ 
times  included,  sometimes  omitted,  and  sometimes  averaged — - 
either  according  to  custom  or  agreement,  or  sometimes  by  the 
1000  brick,  allowing  so  many  to  the  cubic  yard — about  500.  The 
best  dressed  work,  such  as  coping,  cutwaters,  or  raising  stones, 
is  paid  for  either  by  the  cubic  yard  or  the  cubic  foot.  Frame 
timber  is  generally  paid  for  by  the  1000  feet  B.M. ;  B.M.  mean¬ 
ing  board  measure.  The  unit  being  1  foot  B.M.,  that  is,  a  plank 
12  inches  long,  12  inches  wide,  and  1  inch  thick,  a  stick  of  timber 
15  feet  long,  12  inches  broad,  and  10  inches  thick  would  then 
contain  1 50  feet  B.M.  Cross-ties  are  paid  for  at  so  much  apiece. 
Piles  are  generally  paid  for  by  the  lineal  foot,  either  for  the 
ordered  length,  or  for  that  portion  left  in  the  work,  and  allow¬ 
ing  so  much  for  the  cut-off  in  addition. 

268.  Trestle-work  is  sometimes  paid  for  at  so  much  the 
running  foot  of  a  completed  trestle.  This,  in  many  respects, 
is  the  most  satisfactory  mode  of  estimating.  Iron,  whether  as 
bolts,  rods,  cylinders,  and  screw-piles,  so  much  by  the  pound, 
and,  in  case  of  screw-piles  and  cylinders,  so  much  for  each  foot 
sunk  below  water  or  the  bed  of  the  river. 


122  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 

269.  In  sinking  caissons,  so  much  per  cubic  foot  of  material 
moved,  estimated  by  multiplying  the  bottom  area  by  the  dis¬ 
tance  from  the  surface  of  the  water  or  the  bed  of  the  river  to 
the  lowest  point  reached  by  the  caisson,  or  the  lowest  point  of 
the  foundation  bed. 

270.  Again,  all  of  these  separate  items  on  any  work  may  be 
paid  for  in  a  lump  sum.  The  contractor  is  likely  to  get  the 
best  of  this,  as  he  will  be  pretty  sure  to  make  a  liberal  allowance 
for  contingencies;  and  if  the  estimate  is  anyways  doubtful,  it  is 
a  great  temptation  to  do  poor  work,  and  if  any  loss  occurs,  the 
company  will  in  general  make  it  good,  thereby  making  the 
work  cost  more  than  was  anticipated. 

271.  A  responsible  party  who  gives  a  reasonable  bid  is  in 
general  the  safest  to  accept ;  the  highest  bidders  do  not  want 
the  work  very  much,  and  the  lowest  want  it  too  much. 

272.  Stone-cutting  is  paid  for  sometimes  by  the  actual 
number  of  square  feet  cut,  or  simply  by  the  face  measurement 
of  the  stone  ;  the  finest  dressing  by  the  day,  or  by  the  actual 
surface  cut. 

273.  However  the  work  may  be  done,  the  contract  should 
be  clear  and  distinct,  and  full  as  to  all  essential  conditions  and 
requirements,  avoiding  at  the  same  time  useless  and  onerous 
conditions  in  regard  to  minor  details,  which  are  never  carried 
out,  and  only  give  excuse  for  adding  a  good  percentage  to  the 
profit  arising  legitimately  from  the  work.  Say  in  the  specifica¬ 
tions  what  you  mean  and  mean  what  you  say. 

274.  Contractors  are  generally  required  to  furnish  all  mate¬ 
rials^ — tools,  derricks,  engines,  boilers,  and  everything,  called 
the  plant— necessary  to  carry  on  the  work  properly  and  expe¬ 
ditiously.  Poor  contractors  will  furnish  broken-down  carts  and 
mules,  worn-out  boilers,  engines,  and  pumps,  barrows  and 
tools  in  dilapidated  condition,  and  all  often  in  insufficient 
quantities,  causing  loss  of  money  and  time  to  both  contractor 
and  company.  The  company  should  always  reserve  the  right 
to  supply  the  deficiencies,  if  any  exist,  at  the  contractor’s  ex¬ 
pense. 


THE  COST  OF  WORK. 


125 


Article  XXVI. 

THE  COST  OF  MASONRY  AND  CONCRETE. 

275.  The  following  is  the  actual  cost  of  masonry,  concrete, 
etc.,  in  some  important  works  in  the  writer’s  experience  : 

Per 

Cubic  Yard. 


Granite  piers,  not  including  cement,  13,767  cubic  yards . 

“  “  not  including  cement,  second-class  masonry . 

“  pedestals,  not  including  cement,  first-class  masonry . 

Limestone  piers,  first-class  masonry,  not  including  cement,  5652  cubic 

yards . . 

Limestone  abutments,  second-class  masonry . . . 

coping . 

Granite  arch  stone . 

Brick  in  walls  of  houses,  including  mortar,  $11.00  per  1000 . 

Brick  arch  ring . . . 

Sandstone  piers,  cement  included,  about  2000  cubic  yards . 

“  “  “  “  “  10,000  “  “  . 

Retaining-wall,  rubble,  not  including  cement . 

Brick  wall  piers,  2000  cubic  yards . 

Concrete  in  cribs,  not  including  cement,  18,146  cubic  yards . 

“  in  interior  of  caissons,  not  including  cement, 6036  cubic  yards 
“  in  “  “  “  “  “  “  1500  “  “ 

Box-culvert  masonry . 

Paving . . . . . 


$13 

00 

10 

00 

13 

00 

12 

00 

8 

00 

15 

00 

16 

00 

7 

00 

9 

00 

11 

00 

14 

30 

4 

50 

15 

00 

6 

00 

15 

00 

13 

00 

4 

50 

2 

75 

In  the  above  table  the  cost  of  granite  and  limestone  and 
sandstone  piers  included  all  coping  and  cutwater  stone,  and 
also  the  cement  in  the  sandstone  piers  ;  in  the  granite  and  lime¬ 
stone  piers  the  cement  was  furnished  by  the  railroad  company. 
The  sandstone  in  large  part  was  hauled  over  100  miles  by  rail. 
Sandstone  piers  in  the  same  bridge  built  of  local  stone,  with  a 
fair  profit  to  the  contractor,  only  cost  $10.00  per  cubic  yard. 
$12.00,  instead  of  $14.30,  would  have  been  a  good  price  for  the 
work,  and  could  have  been  done  at  that  figure ;  but  the  presi¬ 
dent  of  the  road  preferred  one  contractor  at  $14.30  to  another 
equally  as  good,  in  the  writer’s  opinion,  at  $12.00.  In  the 


124  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


25,682  cubic  yards  of  concrete  as  above  there  was  used  about 
24,230  barrels  cement ;  the  average  cost  of  this  was  $2.08  per 
barrel.  About  two  thirds  of  this  was  Portland  cement,  of  Alsens, 
London,  and  Saylor’s  American  brand ;  these  cost,  respec¬ 
tively,  $2.75,  $2.80,  and  $2.50  per  barrel.  The  Hoffman  and 
Norton  Rosendale  cost  about  $1.29,  delivered  on  the  work. 

In  the  above  table  the  price  of  brick  in  piers  costing  $15.00 
per  cubic  yard  was  caused  by  the  fact  that  brick  were  very 
scarce  and  difficult  to  get,  and  had  to  be  transported  on  barges 
for  a  long  distance  up  a  very  rapid  river,  and  could  only  be 
transported  during  the  rises  in  the  water.  Under  ordinary 
circumstances  $8.00  per  cubic  yard  would  have  been  ample. 
The  box  culvert  masonry  is  higher  than  usual,  owing  to  small 
quantities  and  long  haul.  The  paving  should  not  usually  cost 
more  than  $2.00,  but  good  paving,  which  is  set  edgewise 
and  properly  laid,  with  the  care  necessary  to  avoid  any  undue 
deflection  or  obstruction  to  the  current,  is  worth  a  fair  price,  as 
to  some  extent  it  may  have  to  act  as  an  inverted  arch.  It  is 
well  to  fill  the  interstices  between  the  stones  with  gravel,  chips 
of  stone,  or  even  sand,  as  it  will  prevent  undermining.  The 
flow  of  the  stream  itself  is  likely  to  do  this  filling  during  the 
rises  in  the  stream.  Careless  paving  is  worth  but  little,  and 
owing  to  its  supposed  want  of  importance  is  often  hardly 
worthy  of  the  name.  It  should  be  at  least  from  10  to  12 
inches  thick,  and  should  be  laid  slightly  concave  on  top,  the 
edges  a  little  higher  than  the  centre.  It  is  well  also  for  this 
paving  to  extend  well  beyond  the  apron  walls,  or  even  to  the 
end  of  the  wings.  It  should  never  be  placed  under  the  abut¬ 
ment  or  wing  walls,  but  between  and  abutting  against  them 
well  above  the  bed. 

Cost  of  Quarrying,  Dressing,  Laying,  Finishing,  and 
all  Tools,  Machinery,  and  Materials. 

276.  Cost  of  quarrying  varies  greatly  with  the  kind  of  stone, 
the  condition  of  the  quarry,  cost  of  labor,  distance  of  quarry, 
but  may  be  roughly  estimated. 


THE  COST  OF  WORK. 


125 


Granite.  Sandstone. 

Good  size  stone  for  rubble .  $1.00  per  cu.  yd. ;  $0.50 

Good  size  ashlar  stone .  $4.00  “  “  $3.00 

Dressing  beds  and  joints . $0.30  “  sq.ft.;  $0.15 

Cement  and  sand . $1.00  to  $0.50  per  cu.  yd.;  $1.00  to  $0.50 

Laying .  $2.50  to  $1.75  “  “  $2.50  to  $1.75 

Hauling  depending  on  distance .  $1.00  to  $2.00  “  “  $1.00  to  $2.00 

At  the  above  rate  a  cubic  yard  of  face  or  dressed  stone 
would  cost  in  the  pier  for  granite  $18.00  per  yard,  assuming  35 
square  feet  of  dressed  surface.  The  backing  would  cost  the 
same  as  the  above,  less  the  cutting,  but  allowing  $1.00  for  ham¬ 
mering  and  $1.00  for  the  mortar,  or  $8.50.  In  a  pier  containing 
1500  cubic  yards,  one  third  would  be  cut  stone  and  two  thirds 
backing.  Actual  cost  per  cubic  yard  would  be  $11.60,  pro¬ 
vided  the  cement  was  furnished  by  the  company,  which  is 
a  good  plan  in  many  respects,  but  is  apt  to  lead  to  a  liberal 
use  of  cement ;  but  the  satisfaction  is  that  you  will  insure  better 
cement  and  better  work. 

277.  The  writer  in  this  calculation  assumed  a  stone  6  ft. 
X  2\  ft.  X  2  ft.  just  making  one  cubic  yard,  and  assumed  the 
beds  and  joints  to  be  cut  true  throughout,  the  face  left  rough. 
Stones  of  this  size  are  rarely  of  such  perfect  shape.  The  joints 
of  the  stones  are  not  cut  true  more  than  from  1  to  i\  feet  from 
the  surface.  The  upper  bed  is  not  required  to  be  cut  with  the 
same  nicety  and  care  as  the  lower  bed,  and  it  is  far  better  to  be 
rigid  in  regard  to  the  lower  bed  and  allow  if  necessary  the 
upper  bed  to  be  a  little  rougher  towards  the  back  or  the  tail  of 
the  stone,  for  by  this  means  you  insure  solid  work  and  avoid 
to  a  large  extent  cheating,  consequently  bad  work  somewhere 
in  the  structure ;  in  this  way  you  may  save  three  or  four 
square  feet  of  dressing,  which  is  equivalent  to  from  90  cents  to 
$1.20  per  cubic  yard,  or  the  actual  cost  of  the  work  will  not 
exceed  $10.50  per  cubic  yard,  and  for  $12.60  should  allow  20 
per  cent  profit  to  the  contractor.  Good  first-class  granite 
piers  can  be  built  for  at  most  $13.00  per  cubic  yard.  The 
cost  of  first-class  sandstone  ashlar  would  be  on  the  same 
basis  of  calculation  $9.50  per  cubic  yard.  The  actual  cost 


126 


A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


would  be  $7.90  and  allowing  20  per  cent  profit  to  contractor, 
or  $9.48  per  cubic  yard  of  masonry  in  the  pier.  Limestone 
masonry  would  not  materially  differ  in  cost  from  that  of  sand¬ 
stone. 

278.  Good  brick-work  in  walls  of  buildings  can  be  con¬ 
structed  for  $7.00  per  cubic  yard,  and  from  $10.00  to  $13.00 
per  1000.  In  tunnels,  from  $8.00  to  $9.00.  First-class  masonry, 
$10.00  to  $12.00.  Second-class  masonry  or  brick-work  for  arches, 
$8.50.  Box-culvert  masonry,  from  $2.00  to  $5.00.  Concretefrom 
$4.00  to  $8.00,  varying  largely  in  proportion  to  the  kind  of 
cement  used  and  proportions  of  sand  and  cement,  and  nature 
of  the  broken  stone  used.  Rubble,  from  $3.50  to  $5.00. 
Paving,  from  $1.00  to  $2.00.  Sand  will  cost,  according  to  qual¬ 
ity  and  length  of  haul,  quantity,  etc.,  from  20  cents  to  $1.00. 
Cement,  ordinary,  in  barrels,  from  $1.00  to  $1.25  per  barrel ;  in 
bags,  about  10  cents  less,  say  90  cents  to  $1.1 5.  Portland  cement, 
from  $2.00  to  $3.00  per  barrel.  Brick  cost  from  $6.00  to  $8.00 
per  1000  brick,  according  to  quality  and  demand. 

Article  XXVII. 

DIMENSIONS  OF  PIERS. 

279.  The  following  are  some  of  the  dimensions  and  ffirms 
of  piers  and  abutments  constructed  by  the  writer : 

The  Susquehanna  River  bridge  was  about  6200  feet  long, 
arranged  as  follows:  2  spans,  1  through  and  1  deck,  520  feet ;  4 
deck  spans  480  feet,  2  through  spans  375  feet,  1  deck  span  200 
feet.  There  were  eleven  piers  and  two  abutments.  Six  of 
these  piers  were  in  water  and  five  on  land.  Of  those  in  water 
five  rested  on  pneumatic  caissons  sunk  from  60  to  90  feet  below 
the  water  surface  ;  one  built  inside  a  coffer-dam  ;  all  founded  on 
rock,  except  two,  which  rested  on  beds  of  large  bowlders  mixed 
with  gravel  and  sand  about  70  feet  below  water  surface.  The 
masonry  commenced  on  the  crib  at  varying  depths  below  the 
water  surface,  and  was  built  up  in  steps  or  offsets  to  a  point 
about  4  feet  below  low-water,  at  which  level  the  neat  work  com- 


DIMENSIONS  OF  PIERS. 


127 


menced,  and  was  carried  up  to  the  proper  heights  above  high- 
water,  which  for  the  piers  carrying  the  520-foot  spans  were  90  feet, 
and  for  the  others  or  deck  spans  the  tops  of  the  piers  were  lower 
by  the  depth  of  the  truss,  from  40  to  50  feet.  The  piers  were 
generally  under  coping  32  feet  long  and  10  feet  wide  for  the  low 
piers,  and  35  feet  long  and  11  feet  wide  for  the  four  high  piers 
carrying  the  through  spans.  The  batter  was  ^  inch  to  the  vertical 
foot  from  the  top  to  the  footing-courses  on  both  sides  and  lower 
end.  On  the  upper  end,  about  12  to  1 5  feet  above  low-water,  a 
cutwater  commenced,  sloping  downwards  at  an  angle  of  45 
degrees,  so  that  the  cross-section  of  the  pier  at  the  top  of  the 
footing-courses  would  be  about  20  feet  by  61  feet  long,  to  which 
the  offsets  would  add  about  10  feet  all  round.  With  the  excep¬ 
tion  of  the  upper  end  of  these  piers  to  the  top  of  the  cutwater, 
these  piers  were  square-ended  from  the  bottom  to  the  top. 
The  cutwater  was  finished  with  a  blunt  triangular  end.  The 
coping  and  the  triangular  ends  were  cut  to  a  smooth  surface; 
the  other  parts  of  the  piers  were  first-class  ashlar  masonry,  rock 
face,  with  pitch  line  on  the  joints  or  edges  of  the  stones.  All 
of  these  piers  had  large  raising  stones  on  top  of  the  coping 
6  ft.  X  6  ft.  X  22  in.  See  Plate  XIX,  Figs.  1  and  2. 

280.  There  was  in  addition  about  2300  linear  feet  of  iron 
viaduct  divided  into  30-foot  spans  requiring  about  1 54  pedestals 
reaching  only  a  few  feet  above  the  surface  of  the  ground.  The 
pedestals  were  3%  feet  square  under  coping ;  coping-stone  4 
feet  square  and  15  inches  thick,  projecting  3  inches  over  shaft, 
the  trestle  being  from  40  to  60  feet  high. 

281.  The  total  cost  of  this  bridge  was  $1,737,266,  as  follows  : 
Foundations,  $469,066;  masonry,  $208,000;  superstructure, 
$1,060,200  ;  and  it  was  completed  in  two  years  from  the  time  of 
letting  contract.  All  things  considered,  it  can  be  considered  as 
executed  both  economically  and  expeditiously.  All  masonry 
was  constructed  of  granite  obtained  from  quarries  a  few  miles 
above  the  site  of  the  bridge.  The  raising  stones  were  brought 
from  near  Wilmington,  Del.,  as  stones  of  the  size  required 
could  not  be  obtained  from  the  other  quarry  ;  and  in  addition 
the  Port  Deposit  granite  had  certain  seams  crossing  the  natu- 


128 


A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


ral  beds,  which  rendered  it  uncertain  for  such  heavy  concen¬ 
trated  loads.  The  total  quantities  of  materials  used  in  this 
bridge,  exclusive  of  superstructure,  were  as  follows: 


282.  TABLE  OF  QUANTITIES  AND  COSTS. 

Timber  in  caissons,  cribs,  and 

coffer-dams . 

2,727,755  ft.  B.M. 

@  $46.80  = 

$127,658.93 

Iron  in  caissons,  cribs,  and  coffer¬ 
dams: 

Screw  bolts . . 

124,306  lbs. 

0.06 

7,458.36 

Drift  “  . 

216,028  “ 

0.05 

10,801.40' 

Spikes . 

44,650  “ 

0.054 

2,455-75 

Cast  washers . 

15,206  “ 

O.Q2\ 

323,13 

Total  concrete  in  air-chamber  and  in 

excavations  below  cutting  edge 

4,036  cu.  yds. 

15.00 

60,537-45 

Total  concrete  in  cribs  and  under 

piers . 

11,141  “ 

6.00 

66,846.00 

Excavation,  sinking  caisson  to  cut- 

ting  edge . 

781,934  cu.  ft. 

0.20 

156,386  80 

Excavation,  sinking  caisson  below 

cutting  edge . 

34,452 

0.20 

6,890.40- 

No.  of  bbls.  Portland  cement . 

10,620  bbls. 

2.80 

20,736.00- 

“  “  “  Rosendale  and  Cum- 

berland  cement . 

3,668  “ 

1.29 

4,731-72 

Total  for  foundations . 

$473,825.94 

“  “  Masonry,  first-class . 

14,582.80  cu.  yd 

s.  13.00 

$189,576.40 

“  pedestal  masonry,  first-class. 

429,53 

13.00 

5.583-99 

“  for  masonry,  second-class.... 

817.00  “ 

10.00 

8, 1 70.00 

“  rubble  masonry  and  concrete. 

1,314.61  “ 

6.00 

7,887.66 

“  for  substructure . 

$685,043.89- 

Coffer-dam  for  pier  5,  estimated. . . . 

5,000.00 

Cement  in  masonry  and  concrete. . 

8,700  bbls. 

1.29 

11,143.00 

Cost  of  engineering,  approximate... 

20,000.00 

Extra  bills  handling  material,  extra 

work,  etc.,  estimated . 

20,000  00 

$741, 186.89 

This  includes  some  items  not  included  in  paragraph  281. 

283.  To  determine  the  cost  per  cubic  yard  of  the  volume 
whose  base  is  the  area  of  the  bottom  of  the  caisson  and  whose 
height  is  the  depth  sunk,  which  is  the  most  convenient  form 
for  arriving  at  an  approximate  estimate  of  the  probable  cost 


DIMENSIONS  OF  PIERS. 


129 


of  any  proposed  structure  of  this  kind,  we  will  take  each  cais¬ 
son  separately,  as  all  the  distances  sunk  differ,  and  also  the 
dimensions  of  the  caissons.  We  have  then  the  following  for 
the  above  structure  : 


Dimensions 
at  Bottom. 

Area  in 
sq.  ft. 

Depth 
in  feet. 

Volume 

in 

cu.  ft. 

Volume 

in 

cu.  yds. 

Cost 

per 

cu.  yd. 

Total  Cost. 

Caisson  No.  2.. 

63.27X25.93 

68.32 

1 1 2, 1 24 

4153 

$15.08 

$62,613.41 

i  t 

“  3-- 

67.27X25.93 

70. 72 

65.25 

123,402 

4571 

15-25 

69,603.92 

(i 

“  4-. 

79.40X32.85 

59-9 

159,588 

5911 

15-03 

88,830.64 

88.4 

(t 

“  8.. 

7O.85X32.6l 

76.00 

189,578 

7021 

15-56 

109,248. 70 

78.26 

<» 

“  9-  • 

78.19X42.27 

65.01 

231,692 

8581 

16. 17 

138,769.42 

30237 

15-50 

469,066.09 

The  total  cost  in  this  table  should  agree  exactly  with  the 
corresponding  item  in  preceding  table,  viz.,  $473,825.94;  but  in 
the  above  table  the  concrete  is  calculated  by  averaging  the  cost 
of  cement,  and  in  addition  there  is  some  200  yds.  of  concrete 
under  one  of  the  piers  not  included  in  the  caissons  proper.  The 
iron  is  also  taken  at  an  average  price  in  the  above.  The  above 
table  is,  however,  a  close  approximation  to  the  actual  costs. 
If  the  displacement  is  measured  from  the  bed  of  the  river  and 
not  from  the  water  surface,  the  average  cost  per  cubic  yard  on 
the  above  unit  prices  would  be  considerably  greater  than  the 
above.  As  for  example,  in  caisson  No.  2  the  displacement  would 
be  only  94,504  cu.  ft.  instead  of  1 12,124  cu-  ft.,  and  3500  cu.  yds. 
instead  of  4153  cu.  yds.,  making  the  cost  per  cubic  yard  $17.89 
instead  of  $1 5.08  per  cubic  yard.  These  would  depend  upon  the 
terms  of  the  contract.  In  this  bridge  the  excavation  or  dis¬ 
placement  was  measured  from  the  water  surface.  Mr.  Baker 
in  his  work  makes  this  $19.93  per  cubic  yard,  and  the  average  for 
the  entire  work  $22.69,  instead  of  $1 5. 50,  as  in  table.  The  above 
quantities  and  costs  are  taken  from  the  writer’s  final  estimates 
on  the  work.  In  caissons  4,  8,  and  9  the  actual  depths  to  the 
bottom  of  the  cutting  edges  are  respectively,  59.9,  76.00,  65.01  ; 


130 


A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


whereas  to  the  lowrest  point  of  rock  the  depths  are,  respec¬ 
tively,  65.25,  88.4,  and  78.26.  In  these  caissons  the  cutting  edges 
rested  on  rock  at  one  end,  and  were,  respectively,  5.35,  12.4,  and 
13.25  above  rock  at  the  other.  This  will  again  be  referred  to 
in  discussing  pneumatic  caissons. 


THE  SCHUYLKILL  RIVER  BRIDGE,  B.  &  O.  RY. 

284.  This  bridge,  located  near  Gray’s  Ferry,  Philadelphia, 
was  comparatively  short  and  low,  requiring  a  drawbridge. 
There  were  two  abutments,  three  piers,  and  one  pivot  pier. 
The  spans  were  comparatively  short,  being  as  follows  :  One 
span  201  ft.;  draw-span  242.64  end  to  end,  75  ft.  clear  opening 
at  low-water;  one  span  200  ft.,  one  span  152  ft.  The  line 
crossed  the  river  at  an  angle  of  530  15'  with  the  direction  of 
the  current,  requiring  the  piers  and  abutments  to  be  very  long 
in  proportion  to  the  length  of  the  span.  The  east  abutment, 
U-shaped  in  plan,  was  founded  on  rock  only  a  few  feet  below 
the  surface  of  the  ground.  Pier  No.  5  was  located  on  the  edge 
of  a  rapidly  dipping  rock,  and  was  built  inside  of  a  coffer-dam; 
the  rock  on  the  east  side  was  exposed  at  low-water,  and  on  the 
west  side  was  from  10  to  15  ft.  below  the  water  surface.  The 
range  of  the  tide  was  from  5  to  10  ft.  An  ordinary  coffer-dam 
was  first  tried,  but  owing  to  the  great  difference  of  the  depth 
on  the  two  sides  of  the  dam,  and  the  silty  nature  of  the  material 
overlaying  the  rock  on  one  side,  and  no  material  on  the  other, 
this  dam  failed :  a  good  crib-dam  would  have  stood,  but  on 
such  a  sloping  surface  it  would  have  been  difficult  to  frame 
and  handle.  After  the  failure  of  the  first  dam,  a  contract  was 
made  with  Mr.  J.  E.  Roninson  to  put  in  his  patent  coffer-dam 
(this  will  be  explained  under  Coffer-dams),  and  after  much 
delay  and  many  breaks  we  finally  reached  the  rock.  The 
remaining  piers,  2,  3,  and  4,  rested  on  pneumatic  caissons  sunk 
to  the  rock.  The  west  abutment,  U-shape  in  plan,  rested  on  a 
pneumatic  caisson.  The  following  is  a  table  of  quantities  and 
costs : 


Table  of  Quantities  and  Costs. 


DIMENSIONS  OF  PIERS. 


131 


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132  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 

285.  In  the  above  table  the  price  for  timber  is  taken  at 
$40.00  per  1000  B.M.  This  is  an  average  of  the  prices.  The 
actual  contract  price  was  $45.00  for  timber  in  caissons,  $38.00 
in  cribs  and  coffer-dams,  and  $35.00  for  timber  in  open  caissons 
sunk  on  top  of  piles  for  the  guard  piers.  The  cost  of  iron  is 
also  an  average  for  the  different  grades.  Taking  the  four 
pneumatic  caissons,  exclusive  of  the  masonry,  the  cost  would  be 
$239,047— $34, 364-l-cement  $7489=$2i2,i72.26.  The  total  dis¬ 
placement  in  cubic  yards  is  14,764;  we  find  that  $212,172.26 -r- 
14,764  =  $14.40,  which  is  the  cost  per  cubic  yard  under  the 
water  surface.  This  is  generally  the  basis  upon  which  the  con¬ 
tract  for  piers  is  determined.  In  the  above  case  if  the  excava¬ 
tion  or  displacement  is  taken  from  the  mud  line  the  cost  would 
be  about  $18.00  per  cubic  yard.  It  is  always  better  to  make  the 
water  surface  the  starting  point,  as  in  all  cases  the  low-water  sur¬ 
face  can  be  definitely  ascertained  or  agreed  upon,  marked,  and 
preserved  as  a  datum.  The  bed  of  the  river  may  fill  up  or  scour 
out,  and  the  datum  will  be  uncertain  and  cause  confusion  and 
uncertainty.  The  timber  used  in  these  caissons  was  entirely 
of  yellow  pine.  The  masonry  was  of  a  tough  limestone,  easily 
splitting  in  one  direction,  but  very  difficult  to  split  in  the  other. 
The  pivot  pier  was  33  ft.  in  diameter  under  coping,  35  ft.  at 
bottom  of  shaft,  two  offset  courses,  and  38  ft.  at  bottom  of 
offset  courses.  The  coping  was  16  in.  thick.  The  other 
dimensions  in  the  table  are  given  under  the  coping  and  at  the 
bottom  of  offset  courses. 

OHIO  RIVER  BRIDGE,  POINT  PLEASANT,  W.  VA. 

286.  From  bank  to  bank  on  this  river  was  1370  feet,  divided 
into  five  spans,  respectively  250,  250,  250,  420,  and  200  feet,  by 
six  piers,  two  on  land  and  four  in  the  water.  For  the  land 
piers  pits  were  dug  about  15  feet  deep,  and  piles  driven 
in  the  bottom,  2^  feet  centres,  cut  off  about  1  foot  above  the 
bottom,  then  capped  with  12  X  12  inch  pine,  the  intervening 
spaces  filled  with  concrete;  this  was  then  covered  with  a  solid 
flooring  of  12  X  12  inches,  upon  which  the  masonry  com- 


DIMENSIONS  OF  PIERS. 


133 


menced  ;  concrete  was  piled  up  all  around  from  the  bottom 
to  a  point  about  3  feet  on  the  masonry.  For  the  four  river 
piers  ordinary  coffer-dams  were  constructed,  and  on  the  inside 
of  these  a  timber  crib  was  sunk.  This  work  was  prosecuted 
with  great  vigor  and  in  the  face  of  many  difficulties,  such  as 
floods,  intensely  cold  weather,  and  a  suspension  of  the  work 
for  about  six  weeks  during  the  lowest  water  and  most  favor¬ 
able  weather.  The  average  depth  excavated  below  the  bed  of 
the  river  was  from  10  to  12  feet,  through  gravel  and  sand. 
The  dams  were  strong  enough  to  stand  a  rise  of  15  feet,  or  a 
total  pressure  due  from  25  or  30  feet.  All  of  the  foundations 
were  completed,  and  in  addition  all  the  masonry  of  the  piers, 
within  12  months  from  time  of  commencing,  and  a  consider¬ 
able  number  of  the  pedestals  for  the  iron  viaduct  were  also 
completed. 

287.  The  iron  viaduct  was  2380  lineal  feet,  and  of  a  height 
varying  from  60  feet  to  20  feet.  The  grade  on  the  iron  trestle 
was  feet  in  100  on  both  sides  of  the  river.  The  bridge 
itself  was  constructed  on  a  0.5  grade  on  the  east  and  0.25  on 
the  west  of  the  channel  span.  These  piers  were  built  entirely 
of  sandstone,  mainly  from  the  Hocking  Valley  quarries,  a 
hundred  miles  distant  by  rail,  and  partly  from  a  local  quarry, 
called  Miller’s  Quarry.  The  following  table  gives  the  crushing 
strength  of  true  cubes,  2X2X2  inches,  using  in  the  crushing 


Slight  Signs  of 

Crushed  or  Split. 

Location  of  Quarry. 

Yielding  at  Press¬ 
ure  per  cube ; 
per  square  inch. 

Pres, 
pr.cube 
in  lbs. 

Pres.  pr. 
sq.  in. 
in  lbs. 

Hocking  Valley,  No.  1. 

“  “  2. 

<<  <<  <i  - 

“  “  “  4! 

“  “  “  5- 

“  “  *  6. 

4,800  or  1,200 

23,526  “  5,831^ 
6,650  “  1,662! 
8,130  “  2,032! 

17,620  “  4,405 

18,740  “  4,685 

18,458 

27,885 

12,000 

15,930 

17,620 

18,740 

14,942 

15,442 

4,614 

6,971 

(  Without  violence  or 
}  noise. 

Slight  noise. 

Not  crushed. 

j  Crushed  suddenly 
(  without  noise. 

(  No  evidence  of  yield- 
(  ing  whatever. 

3,982 

4,405 

4,685 

3,735 

3,860 

“  “  '  “  2 

134 


A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


soft  white-pine  cushions.  These  cubes  were  dressed  true  in  a 
marble  yard. 

The  writer  tested  these  specimens  with  the  above  results, 
and  also  many  other  specimens  from  various  places;  the 
general  average  was  the  same  as  above.  The  piers  were  built 
of  the  above  stone.  The  tests  were  made  at  the  Ohio  State 
University,  Columbus.  These  specimens  were  all  compara¬ 
tively  fresh  from  the  quarries,  as  we  could  not  wait  very  long 
before  deciding  upon  the  quarry.  The  specimens  would 
doubtlessly  have  resisted  a  higher  pressure  after  seasoning 
thoroughly.  Pasteboard  cushions  are  now  recommended  as 
better  than  pine.  The  following  are  the  quantities  and  costs: 

273,210  feet  B.M.  pine  timber  in  coffer-dams,  cribs,  and  foundations  under  piers* 
244,412  “  “  oak  “  “  “  “  main  and  sheet  piling. 

3,597  “  “  poplar  “  “  “  “  sheet  piling. 

1 3, 5 71  lineal  feet  of  piles  in  foundations  and  coffer-dams. 

3,499  cubic  yards  excavation  sand  and  gravel. 

649  “  “  concrete  in  foundations. 

997  rip-rap  stone. 


Total  cost  of  foundations .  $64,652  62 

Masonry  in  piers,  first-class,  8,654  cubic  yards,  $14.30 .  123,756  92 

Masonry  in  pedestals,  “  “  1,224  “  “  14-30 .  I5>912  00 

Earth  in  approaches,  39,490  “  “  .22 .  8,687  80 

“  “  pedestal  foundations .  2,304  02 

371,962  feet  B.M.  in  trestle  approach,  $30  per  1000  feet  B.M .  11,158  86 

11,965  vvrought-iron  screw  and  drift  bolts,  5  cts .  598  25 

7,939  cast-iron  washers  and  packing  spools,  4  cts .  317  56 

Extra  bills .  J74  64 


Total  cost  of  substructure  and  approaches,  exclusive  of  iron  viaduct  $227,562  67 

Total  cost  of  bridge  proper,  1370  lineal  feet,  $126.50 .  173.213  00 

“  “  “  iron  viaduct,  2380  “  “  39°° .  92.771  °o 


Total  cost  of  completed  structure .  $493,540  67 


288.  The  dimensions  of  piers  and  pedestals  were  as  fol¬ 
lows  :  Pier  No.  1,  carrying  one  end  of  one  span  250  feet 
and  end  of  iron  viaduct,  at  top  22.22  X  6.5  feet,  at  bottom 
25.54  X  10.04  feet,  43.84  feet  high.  Offset  course  4.65  feet, 
height,  and  28.53  X  1 2.53  feet  at  bottom,  total  height  48.49  feet. 


DIMENSIONS  OF  PIERS. 


135 


square  ends.  Piers  2  and  3,  carrying  250-feet  spans,  top  23.00 
X  9.00  feet,  square  ends,  at  33.35  feet  from  top,  25.8  X  11.8  ft. 
Belt-course  2  feet  thick,  38.92  X  13.22  feet,  projecting  9  inches 
all  around.  The  piers  were  lengthened  at  the  belt-course  by 
adding  semicircular  ends  from  that  point  to  below  low-water. 
Main  wall  under  belt-course  37.42  X  1 1.72  feet,  bottom  of  neat 
work  42.20  X  16.50  feet,  height  59.39  feet.  Then  four  offset 
courses  8.58  feet  thick,  bottom  dimensions  49.00  X  23.00  feet. 
Total  height  97.32  feet.  Piers  4  and  5,  carrying  channel  span 
420  feet,  top  26.25  X  10.55  feet>  at  top  of  belt-course  28.88  X  1 3.20 
feet.  Main  wall  under  belt  course  42.24  X  13.36  feet.  Bottom 
neat  work  46.92  X  18.04  feet.  Three  offset  courses  5.6  feet 
high.  Bottom  dimensions  51.72  X  22.84  feet.  Total  height 
96.86  feet  fof  No.  4  and  101.60  feet  for  No.  5.  Square  ends 
to  belt-course,  rounded  ends  to  bottom.  Pier  No.  6,  carrying 
200-feet  span,  top  22.00x6.00  feet,  bottom  of  neat  work  27X11 
feet,  height  of  shaft  59.88  feet.  Two  offset  courses  5  feet 
thick,  bottom  dimensions  31.06  X  14.60  feet.  Total  height 
64.88  feet.  This  case  is  entered  into  as  well  illustrating  a 
good  standard  of  dimensions  and  form  of  piers.  Minimum 
dimensions  for  piers  carrying  length  of  spans  above  called  for. 
All  of  these  piers  had  raising  stones  of  Berea,  Ohio,  sandstone, 
a  hard,  strong  stone.  The  coping  was  doubled,  the  bottom 
course  projecting  9  inches  all  around.  Thickness  of  each 
course  was  18  inches.  A  cone-shaped  finish  was  placed  at  the 
ends  on  top  of  the  belt-course  in  passing  from  the  curved  ends 
to  the  square  ends.  The  pedestals  were  4  feet  square  under 
coping,  coping  1 5  inches  thick,  projecting  3  inches.  Top  of  ped¬ 
estal  was  from  2  to  4  feet  above  ground.  Two  offset  courses 
below  ground,  generally  built  of  only  two  stones  to  the  course, 
sometimes  three  stones  allowed  in  the  footing-courses.  It  is 
best  to  arrange  elevation  of  the  top  of  the  coping,  where  the 
ground  will  allow,  so  as  to  have  as  many  trestle-bents  of  the 
same  height  as  possible. 

For  elevation  and  plan  of  pier  as  just  described,  see  Plate 
I,  Figs.  1  and  2. 


136  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


Article  XXVIII. 

DEFINITIONS. 

PARTS  OF  THE  ARCH. 

289.  Abutment. — The  masonry  supports  of  the  arch  ring. 

Skew-back. — A  course  of  stone  on  top  of  the  abutment, 
with  an  inclined  surface  from  which  the  arch  directly  springs. 

Arch  Ring. — The  masonry  of  the  arch  between  the  intrados 
and  extrados. 

Intrados  or  Soffit. — The  under  curved  surface  of  the  arch 
ring. 

Extrados  or  Back. — The  top  curved  surface  of  the  arch 
ring. 

Crown. — The  highest  part  of  the  arch. 

Springing  Line. — The  line  on  the  soffit  at  the  top  of  the 
abutment. 

Haunches. — The  lower  part  of  the  arch  ring  on  both  sides. 

Spandrels. — A  wall,  or  walls,  built  on  the  top  of  the  arch, 
commonly  one  at  each  end  and  in  the  plane  of  the  face,  2  to  5 
feet  high. 

Span. — The  horizontal  distance  between  springing  lines. 

Rise. — The  vertical  distance  from  the  springing  to  intrados 
at  the  crown. 

King-stones. — A  course  of  stones  between  two  vertical 
planes;  does  not  actually  exist,  as  the  stones  break  joints, 
there  being  no  continuous  joint  in  a  plane  parallel  to  the  face 
of  the  arch.  The  face  stones,  or  those  seen  on  the  ends  of  the 
arch,  are  frequently  cut  on  top  with  a  horizontal  and  vertical 
surface  which  project  above  the  extrados. 

String-course. — A  course  of  stone  extending  from  end  to 
end  of  the  arch. 

String-course  Joint. — The  joints  between  the  string-courses, 
continuous  and  in  the  same  plane  from  end  to  end  of  arch. 

Semicircular  or  Full  Centre  Arch. — One  in  which  the  in¬ 
trados  is  a  full  half  of  the  surface  of  a  cylinder. 


DEFINITIONS. 


137 


Segmental  Arch. — One  in  which  the  intrados  is  less  than 
the  surface  of  a  semi-cylinder. 

Elliptical  Arch. — One  in  which  the  intrados  is  part  of  an 
•elliptic  cylinder  ;  one  in  which  the  rise  is  less  than  the  half-span. 

Pointed  Arch. — One  in  which  the  rise  is  greater  than  the 
half-span,  generally  formed  by  the  intersection  of  two  equal 
•circles. 


DEFINITION  OF  PARTS  OF  PIERS,  RETAINING-WALLS,  ETC., 

290.  Face. — The  exposed  part  of  a  pier  or  wall. 

Facing  Stone. — The  stones  that  show  on  the  face  of  the 
wall. 

Backing. — The  stones  behind  the  facing  stones  in  retaining- 
walls  and  between  the  face  walls  in  piers,  and  well  bonded  to 
the  face  stones.  Also  called  filling,  whether  of  large  stones, 
rubble,  or  concrete. 

Batter. — The  inclination  of  the  face  of  a  wall  to  a  vertical, 
generally  expressed  in  fractions  of  the  height,  as  ^  inch,  1  inch, 
1^  inch  to  1  foot  vertical ;  ordinarily  ^  inch  to  1  foot. 

Bond. — The  overlapping  of  the  stones  so  as  to  tie  the  wall 
together. 

Course. — A  layer  of  stone  between  two  horizontal  planes  or 
joints. 

Joints. — The  space  between  the  stones,  generally  filled  with 
mortar.  The  bed-joints  are  the  top  and  the  bottom,  generally 
horizontal;  and  the  side-joints,  which  are  either  vertical  or 
inclined,  generally  vertical. 

Stretcher. — A  stone  that  shows  its  full  length  on  the  face 
and  all  stones  parallel  to  it  in  the  backing. 

Header. — A  stone  that  shows  its  end  on  the  face  and  all 
parallel  to  it  in  the  backing. 

String  or  Belt  Course. — A  course  of  large  stones,  projecting 
from  6  to  9  inches  from  the  face  of  the  wall ;  generally  dressed 
smooth  on  the  exposed  part,  and  also  has  a  wash  cut  on  it. 
Used  mainly  for  appearances,  and  marks  a  change  from  a 
curved  to  a  plain  finish  to  a  pier. 


1 38  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 

Coping. — A  course  of  large  stones  on  the  top  of  the  pier, 
projecting  from  6  to  9  inches,  dressed  true  on  all  surfaces; 
has  a  wash  on  the  projecting  part.  Sometimes  two  coping- 
courses  are  used.  The  upper  one  has  no  projection,  and  in 
fact  the  lower  coping  projects  beyond  the  upper  6  to  9  inches. 
These  coping-stones  are  commonly  bolted  to  the  pier,  or 
fastened  to  each  other  by  cramps  or  dowels. 

Pedestal  or  Raising  Stones. — Large  thick  stones  of  some 
hard  variety,  placed  on  top  of  coping,  upon  which  the  ends  of 
the  bridge  directly  rest.  These  are  not  always  used. 

Pointing. — Cleaning  out  the  joints  to  the  depth  of  1  to  1^ 
inches,  and  refilling  with  good  mortar. 

Quoins  or  Corner-stones  are  stones  at  the  corner  showing 
header  on  one  face  and  stretcher  on  the  other. 

Dowels.— A  straight  bar  of  stone  or  iron,  fitting  into  holes 
cut  in  the  sides  or  beds  of  adjacent  stones  so  as  to  prevent  one 
lifting  without  the  other. 

Cramps — Iron  bars  18  to  20  inches  long,  bent  at  right 
angles  at  the  end  for  2  to  3  inches  and  placed  across  the  joint, 
the  bent  ends  let  into  holes  cut  in  the  top  of  the  stone ;  a 
groove  also  being  cut  between  the  holes  so  that  the  bar  will 
not  project  above  the  surface  of  the  stone.  Hot  lead,  sulphur, 
or  cement,  is  generally  poured  around  the  bar  to  fasten  it  in 
place. 

291.  The  character  of  the  masonry  is  determined  by  the 
size  of  the  stone,  the  regularity  of  the  courses,  the  amount  of 
dressing  or  cutting. 

Ashlar  Masonry  or  Block-in-course. — -Masonry  laid  in  regular 
courses  with  bed-joints  horizontal  and  side  joints  vertical; 
all  the  stones  cut  into  regular  blocks;  all  surfaces  dressed 
smooth  except  the  face.  This  may  or  may  not  be  dressed. 

Random  or  Coursed  Rubble. — Masonry  in  which  the  stones 
are  cut  to  regular  shapes,  side  joints  vertical,  bed-joints  hor¬ 
izontal  but  not  continuous,  courses  of  varying  thicknesses, 
stones  being  large  and  small,  thick  and  thin. 

Common  Rubble. — Masonry  in  which  the  stones  are  built  as 


DEFINITIONS. 


139 


they  come  from  the  quarry,  with  no  regular  courses  ;  the  joints 
are  not  necessarily  either  vertical  or  horizontal.  Large  and  small 
stones  are  used  at  random  and  with  or  without  mortar. 

Stones.- — The  upper  and  lower  surfaces  are  called  beds,  the 
remaining  parts  are  called  sides,  face,  and  back. 

Quarry-faced. — When  the  face  is  left  as  it  comes  from  the 
quarry. 

Rock-faced , — When  the  face  is  roughly  hammered  so  as 
not  to  project  more  than  from  3  to  5  inches. 

Pitch  Line. — When  a  straight,  well-defined  line  at  the 
angles  is  cut  all  around  the  face  of  the  stone. 

Chisel  Draft. — When  a  smooth,  plane  surface  from  1  to 
i-J  inch  is  cut  around  the  face  of  the  stone,  forming  well- 
defined  and  regular  angles ;  the  rest  of  the  face  left  rough. 

292.  The  face  of  the  stones  may  be  left  rough.  If  the  face 
has  no  projection  over  %  to  f  inch,  it  is  said  to  be  rough- 
pointed  ;  if  the  projections  are  not  over  to  inch,  it  is 
called  fine-pointed,  and  is  what  is  generally  understood  by 
dressed  stone,  whether  for  face  or  for  beds ;  on  the  face  it  is 
made  to  look  uniform  and  regular.  When  required  to  be  of  a 
smoother  surface  than  the  fine-pointed,  it  is  generally  said  to 
be  bush-hammered  ;  this  is,  however,  done  by  an  instrument 
called  the  crandall,  which  consists  of  a  number  of  double- 
pointed  steel  pins  fastened  close  together  in  a  slot  at  the  end 
of  an  iron  bar,  and  produce  a  smooth  and  more  regular  surface 
than  the  fine  point.  The  bush-hammer  is  not  commonly  used 
by  stonecutters.  The  crandall  or  bush-hammer  is  only  required 
for  dressing  coping  or  cutwater  stones.  For  perfectly  smooth 
faces  the  stone  is  first  sawed  and  then  rubbed  to  a  smooth 
surface.  It  is  only  used  for  ornamental  purposes. 

293.  The  common  way  of  raising  large  heavy  stones  to 
their  position  on  the  wall  is  by  means  of  derricks,  which  con¬ 
sist  essentially  of  a  mast  of  greater  or  less  height,  resting  on  a 
solid  block  of  wood  and  a  boom  connected  with  it  at  or  near 
the  bottom,  and  also  by  a  rope  at  the  top.  A  hoisting  rope 
passes  from  a  drum  or  capstan  over  a  sheave  in  the  top  of 


140  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 

the  boom  and  thence  downward,  terminating  in  a  chain  which 
has  hard  steel  hooks  at  the  bottom.  Holes,  for  the  hooks, 
are  cut  into  the  sides  of  the  stone  a  little  above  the  line  passing 
through  its  centre  of  gravity.  This  is  the  common  mode. 
Sometimes  a  dovetailed  mortise  is  cut  into  the  top  of  the  stone, 
thicker  at  the  bottom  than  at  the  surface,  and  a  lewis  made  of 
three  pieces  of  iron,  two  of  which  are  truncated  wedges,  the 
other  rectangular.  The  wedge-shaped  pieces  are  first  inserted, 
and  are  forced  apart  by  driving  the  straight  piece  between 
them.  The  hoisting  chains  are  attached  to  the  wedge-shaped 
pieces,  which  can  not  be  pulled  out  without  breaking  the  stone. 
This,  however,  is  only  applicable  to  hard,  strong  stone,  unless 
the  mortise  is  cut  very  deep.  Another  method  is  to  drill  two 
holes,  in  a  plane  passing  through  the  centre  of  gravity  of  the 
stone,  inclining  towards  each  other  at  an  angle  of  90°,  or  450 
to  the  vertical  on  either  side  ;  strong  iron  bars  are  inserted  in 
these  holes,  and  chains  are  fastened  to  eyes  on  the  other  ends 
of  the  iron  bars. 

For  details  of  common  forms  of  derricks,  see  Plate  V,  Figs. 
5,  6,  and  7;  and  forms  of  derrick  set  on  top  of  pier  and  lifted 
by  screws  as  the  masonry  is  built,  see  Plate  V  A,  Fig.  1. 

Tables  of  Ultimate  Strength  of  Stones,  Natural  and  Artificial, 
to  resist  Crushing,  Tearing,  or  Cross-breaking,  as  given  by 
Several  Authorities,  in  lbs.  per  Square  Inch. 


294.  TABLE  1. 


Resistance  to  Crushing, 
in  lbs.  per  square  inch. 

Rankine. 

Baker. 

Trautwine. 

12,861 

12,000  to  21,000 
8,000  to  20,000 
7,000  to  20,000 
5,000  to  15,000 

6,222  to  12,444 
4,000  to  9,340 
4,000  to  9,340 
2,333  to  6,999 
6,222  to  12,444 
800  to  4,800 
310  to  465 
465  to  1,162 

8,528  to  3,050 
9,824  to  3,000 

Brick . 

1,100 

2,500  to  3,000 
1,150  to  1,290 
1,650  to  1,850 

Brick-work  in  cement.. 

800  to  1,000 

DEFINITIONS. 


141 


TABLE  2. 


Transverse  Strength  or 
Resistance  to  Cross-break¬ 
ing,  in  lbs.  per  square  inch 
Modulus  of  Rupture. 

Rankine. 

Baker. 

Trautwine.* 

goo  to  2,700 
144  to  2,800 

576  to  2,340 

1,800  to  9,000 
269  to  1,796 

900  to  2,700 

Limestone  and  ^ 

360  to  1,260 

3,600  to  5,700 
180  to  540 

Sandstone  j 

Brick . 

200  to  380 

295.  Although  there  are  considerable  differences  between  the 
resistance  to  crushing  of  the  stones  above  given,  no  inconven¬ 
ience  or  doubt  need  rise  as  to  the  strength  of  any  of  the  above, 
as  200  lbs.  per  square  inch  is  an  unusual  pressure,  and  this  only 
exists  under  the  largest  and  highest  structures,  and  then  only 
when  the  normal  unit  pressure  is  increased  by  wind  pressure 
on  the  leeward  side. 

296.  To  apply  the  table  of  crushing  strength  to  any  struc¬ 
ture,  it  is  only  necessary  to  multiply  the  unit  pressure  in  the 
column  by  the  area  of  tire  cross-section  in  the  same  unit  to 
obtain  the  total  resistance  to  crushing,  as  R  =  pA,  in  which  p 
is  the  unit  or  coefficient  of  resistance  to  crushing  in  lbs.  per 
square  inch  or  per  square  foot,  and  A  is  the  number  of  square 
inches  or  square  feet  in-  the  base  of  the  structure,  and  R  the 
total  resistance  to  crushing.  For  example,  take  the  average 
area  of  a  pier  built  of  sandstone  to  be  22  X  40  =  880  square 
feet.  In  the  column  from  Rankine’s  Engineering  the  least 
resistance  of  sandstone  to  crushing  is  3000  lbs.  per  square  inch 
=  432,000  lbs.  per  square  foot,  or  216  tons  of  2000  lbs. 
per  square  foot,  hence  R  =  pA  216  X  880  =  190,080  tons. 
Now  to  determine  the  height  of  a  sandstone  pier  that 
would  crush  at  the  base  under  its  own  weight  (assuming  that 
it  does  not  give  way  by  flexure  or  transverse  strain,  the  limit 

*  Trautwine  always  takes  the  lengths  of  beams  (/)  in  feet,  in  which  case  the 
moduli  of  rupture  are  only  of  the  numbers  in  this  table.  See  Trautwine, 
page  185. 


142 


A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


of  which  is  generally  taken  at  a  height  not  over  20  times  its 
least  dimension,  a  height  which  rarely  occurs  in  practice.  (The 
height  of  the  Washington  Monument  is  500  feet  to  the  bottom 
of  the  pyramidal  finish,  and  the  least  diameter  of  the  column 
36-2-  at  top,  and  50  ft.  at  bottom;  the  middle  would  be  43^; 
therefore  the  height  is  only  14  times  its  least  diameter.)  The 
ultimate  crushing  strength  is  216  tons  per  square  foot,  or 
432,000  lbs. ;  assuming  that  the  weight  of  sandstone  masonry 
is  140  lbs.  per  cubic  foot,  we  have  140  X  y  —  432,000,  hence 
the  required  height  y  =  3685  ;  and  in  the  same  manner  the 
height  of  a  brick  column,  assuming  brick  to  weigh  125  lbs.  per 
cubic  foot,  would  vary  from  600  to  900  ft.  to  crush  of  its  own 
weight,  and  a  granite  pier  about  8000  ft.  high.  In  selecting 
the  unit  of  resistance  to  crushing  from  the  tables,  whether  you 
take  the  least  or  the  average  or  the  greatest,  depends  upon 
the  kind  or  quality  of  the  stone  considered.  (Table  1.) 

297.  The  base  of  a  brick  chimney  at  Glasgow,  Scotland, 
468  feet  high,  bears  9  tons  per  square  foot,  and  in  high  winds 
may  have  to  bear  as  much  as  15  tons  per  square  foot,  or  210 
lbs.  per  square  inch  at  base  ;  and  Mr.  Trautwine  expresses  the 
opinion  that  first-rate  hard  brick  laid  in  cement  would  carry 
without  completely  cracking  100  tons  per  square  foot  or  1400 
lbs.  per  square  inch. 

298.  So  far  as  stone  is  concerned,  the  main  application  of 
the  table  of  transverse  strength  is  in  the  case  of  lintels  over 
openings,  which  in  general  are  to  be  considered  as  beams  uni¬ 
formly  loaded  ;  but  as  they  may  sometimes  also  be  subjected 
to  a  single  concentrated  load  at  the  centre,  it  will  be  well  to 
apply  the  formula  for  both  cases,  although  the  first  is  two  times 
as  sweat  as  the  second.  The  writer  will  use  Rankine’s  formula, 

o 

which  is  easy  to  remember,  when  the  principle  of  moments 
is  understood,  is  easy  of  application,  and  applies  to  all  condi¬ 
tions  of  loading  and  supporting  beams,  which  will  be  further 
explained  in  connection  with  timber.  The  formula  is:  mWl  = 
nfb) 7,  in  which  in  is  a  factor  depending  upon  the  manner  of 
loading  and  supporting  the  beam,  IV  is  the  concentrated  weight 
at  the  middle  point  between  the  supports;  l  is  the  length  of 


DEFINITIONS. 


143 


the  beam  or  clear  span  in  inches  \  n  is  a  factor  depending  on 
the  cross-section  of  the  beam,  and  for  rectangular  beams  is 
equal  to  £ ;  /  is  the  modulus  of  rupture  in  lbs.  taken  from  the 
table ;  b  is  the  breadth  in  inches,  and  h  is  the  depth  in  inches. 
Suppose  a  lintel  to  be  10  inches  thick,  2  feet  wide,  and  10  feet 
long,  loaded  with  a  single  weight  at  the  centre,  the  lintel  to 
be  rectangular  in  cross-section,  and  the  stone  granite ;  then 
.5 m  =  —  120  inches,  n  —  f  —  2700,  as  this  would  always 

be  the  best  and  strongest  stone  ;  b  =  24  inches,  and  h  =  10 
inches.  Substituting  in  the  formula,  we  have 


\W  X  120  —  £ 2700  X  24  X  100; 


4  X  2700  X  24  X  100 
~  6  X  120 


hence  W  —  36,000  lbs.  centre-breaking  load.  When  uniformly 
distributed  over  the  beam,  it  would  be  double  the  above,  or 
72,000  lbs.,  as  will  be  seen  from  the  formula ;  W,  in  this  case, 
=  wl ,  in  which  w  —  weight  on  a  unit  of  length,  and  m  —  i,  all 
other  values  of  same  ;  hence 


\{wt)l  —  ^  2700  X  24  X  100  ;  or, 


wl  — 


8  X  2700  X  24  X  100 
6  X  120 


hence  W  —  wl  —  72,000  lbs.  For  other  materials  and  other 
dimensions  similar  results  can  be  obtained,  using  Table  2  for 
transverse  strength- 

299.  1°  practice,  under  steady  loads,  it  would  not  be  safe 
to  rely  upon  more  than  from  \  to  of  the  above  loads ;  but  it 
is  safer  to  use  only  from  ^  to  T/  of  the  above  results;  that  is, 
for  a  granite  beam  of  the  dimensions  given  above,  it  should 
not  be  loaded  with  more  than  7200  lbs.,  or  720  lbs.  per  foot  of 
length. 

300.  TABLE  3. 

Table  of  Tensile  Strength  of  Mortar  in  Pounds  per  Square  Inch. 

(From  Baker.) 

Hydraulic,  with  sand,  30  to  300,  age  from  1  week  to  1  year,  1  sand,  1  cement. 
Hydraulic,  <  neat  )  from  40  to  400. 

Cement,  (  cement  )  from  100  to  800,  age  from  1  day  to  1  year. 

Cement  and  sand,  from  80  to  350,  age  from  1  week  to  1  year,  3  sand,  1  cement. 


1 44  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


301.  The  adhesive  strength  of  mortar  varies  greatly  with 
the  kind  of  cement  used  and  the  proportion  of  sand,  the  clean¬ 
ness  of  the  surface  of  the  brick  or  stone,  whether  porous  or  not. 
Mr.  Rankine  gives  15  lbs.  per  square  inch  to  limestone  and  33 
lbs.  per  square  inch  to  brick.  According  to  Baker,  in  Portland 
neat  cement  the  adhesive  strength  varies  for  limestone  from  57 
to  78  lbs.  per  sq.  in.  and  from  19  to  213  lbs.  per  square  inch 
for  brick;  and  when  mixed,  1  cement,  2  sand,  the  adhesive 
strength  varies  from  5  to  140  lbs.  per  square  inch,  according  to 
the  character  of  cementing  material  and  stone  used. 

302.  The  absorptive  power  may  be  taken  as  one  part  for 
from  80  to  700  parts  in  granite,  and  1  part  in  from  30  to  60  of 
sandstone,  limestone  from  1  part  in  20  to  1  part  in  500,  and 
for  brick  from  I  part  in  4  to  I  part  in  50,  and  mortars  from  I 
part  in  2  to  1  part  in  10.  In  general  a  small  absorptive  power 
is  an  indication  of  a  good  quality  of  stone. 

303.  According  to  Rankine,  the  expansion  of  stone  is  as 
follows:  brick,  .00355  °f  its  dimensions;  sandstone,  .0009  to 
.0012;  marble,  .00065  to  .0011  ;  granite,  .0008  to  .0009  of  their 
linear  dimensions  in  a  range  of  180°  Fahr. 

304.  TABLE  4. 

Table  of  Weight  in  Pounds  per  Cubic  Foot  of  many  Substances. 


(From  Trautwine.) 


Granite .  168 

Limestone .  172 

Marble .  172 

Sandstone . 15° 

Slate . . .  175 

Common  brick .  125 

Pressed  brick .  150 

Masonry  of — 

Granite .  165 

Rubble .  125 

Sandstone .  144 

Common  brick . 125 

Pressed  brick .  140 


Sand .  99  to  1 17 

Sand,  packed .  101  to  119 

Sand,  wet .  120  to  140- 

Clay,  dry .  63 

Ordinary  earth .  72  to  92 

Ordinary  earth,  packed. ...  90  to  100 

Mud .  104  to  120 

Hydraulic  cement .  60  to  80 

Portland  cement .  80  to  87 

Mortar,  dry .  100 

Concrete  . 

Water .  62.33 


DEFINITIONS. 


145 


The  above  table  is  useful  in  determining  the  stability  of 
retaining-walls,  weight  of  structures,  and  force  tending  to  over¬ 
turn  the  wall  or  to  cause  sliding. 

305.  The  following  are  the  angles  of  repose  or  the  angles 
of  friction  between  different  substances  heretofore  considered  : 


TABLE  5. 


(From  Trautwine.) 

Polished  marble  on  polished  marble . 

Polished  marble  on  common  brick . 

Common  brick  on  common  brick . 

Common  brick  on  dressed  soft  limestone . 

Common  brick  on  dressed  hard  limestone . 

Hard  limestone  on  dressed  hard  limestone . 

Hard  limestone  on  dressed  soft  limestone . 

Soft  limestone  on  dressed  hard  limestone . 

Masonry  and  brick-work,  dry . 

Masonry  and  brick-work  mortar,  damp . 

Masonry  and  brick-work,  dry  clay . 

Masonry  and  brick-work,  moist . 

Wet  clay . 

Dry  clay . 

Damp  clay . 

Shingle  and  gravel . 


Coefficient 


of  Friction. 

9° 

6' 

O.16 

23° 

45' 

O.44 

32° 

38' 

0.64 

33° 

2' 

0.65 

3i° 

00' 

O.60 

20° 

48' 

0.38 

33° 

50' 

0.67 

33° 

2' 

0.65 

33° 

2' 

0.65 

36° 

30' 

O.74 

27° 

00' 

0.51 

18° 

15' 

0-33 

14° 

to  1 70 

0.25  to  0.31 

21° 

to  370 

0.38  to  0.76 

45° 

00' 

1. 00 

35° 

to  48° 

0.70  to  0.90 

This  table  is  useful  in  determining  stability  of  walls  and 
arches  against  sliding  in  connection  with  weight  of  walls  and 
position  of  plane  of  rupture  in  calculating  the  thrust  exerted 
against  walls.  To  determine  resistance  to  sliding  of  one  body 
on  another  multiply  normal  component  of  weight  of  one  body 
resting  on  another  by  the  coefficient  of  friction,  if  the  surfaces 
are  inclined,  and  the  entire  weight  if  the  surfaces  are  horizon¬ 
tal.  What  is  the  resistance  of  a  block  of  dry  masonry  weighing 
20  to  sliding  on  any  ioint?  20  X  .65  =  13  tons. 


14^  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 

306.  TABLE  6. 

The  Bearing  Power  of  Soils  in  Pounds  per  Square  Inch  and  Tons 
per  Square  Foot. 


Rankine,  Safe  Load. 

Baker,  Safe  Load. 

Ulti¬ 
mate 
Load, 
tons 
per 
sq.  ft. 

Lbs.  per 
sq.  in. 

Tons  per 
sq.  ft. 

Lbs.  per 
sq. in. 

Tons  per 
sq.  ft. 

Clay,  dry . 

17  to  23 

ii  to  I-J 

55  to  86 

4  to  8 

15 

Sand . 

it  4< 

“  “ 

55  to  86 

4  to  6 

15 

Clay  and  sand . 

it  a 

it  ii 

55  to  86 

4  to  6 

Sand  and  gravel . 

it  it 

a  a 

in  to  140 

8  to  10 

Clay,  wet . 

.... 

20  to  30 

to  2 

Layer  of  clay  over  quicksand 

.... 

20  to  40 

i|  to  23 

Alluvial  soil.  New  Orleans. . . 

.... 

7  to  14 

i  to  1 

We  may  then  safely  conclude  that  ordinary  soils  can  be 
easily  loaded  with  from  2  to  3  tons  per  square  foot  or  from 
4000  to  6000  lbs.,  and  for  softer  soils,  or  firm  soils  resting  on 
softer  soils,  2000  to  4000  lbs.  per  square  foot. 


PART  SECOND. 


Article  XXIX. 

TIMBER  FOUNDATIONS. 

Under  this  heading  are  included  simple  timber  foundations; 
piles,  whether  cut  off  under  ground  or  under  water,  as  well  as 
when  left  standing  above  the  surface,  as  is  the  case  in  pile 
trestles ;  framed  trestles  ;  timber  piers  for  bridges  ;  timber  cribs, 
whether  filled  with  concrete  or  broken  stone ;  open  timber 
caissons;  coffer-dams;  Cushing  cylinder  piers ;  etc. 

TIMBER. 

I.  Timber  is  used  extensively  in  the  above  structures  for 
the  following  reasons  :  1st.  As  a  matter  of  economy.  It  is  often 
impossible  to  procure  stone  or  brick  in  any  reasonable  time  or 
cost,  but  timber  of  some  kind  can  commonly  be  procured 
which  will  at  least  do  for  a  structure  of  a  temporary  character; 
and  if  under  water  or  in  wet  or  even  constantly  moist  ground, 
it  can  be  relied  upon  for  the  foundations  of  permanent  struct¬ 
ures,  as  it  will  not  rot  when  constantly  wet.  When  immersed  in 
sea-water  it  is  rapidly  honeycombed  and  destroyed  by  sea-worms, 
unless  creosoted.  2d.  Timber  which,  either  on  account  of  its 
small  dimensions  or  excess  of  sap,  would  be  unsuitable  for 
structures  above  ground,  may  be  suitable  for  those  underwater 
or  under  ground.  3d.  Timber  is  easily  framed  and  handled, 
and  can  be  transported  overland  or  floated  in  large  rafts  on 

147 


I48  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 

rivers  or  streams,  yet  lias  the  strength  to  bear  heavy  loads 
and  strains.  But  be  sure  that  the  timber  will  always  remain 
wet. 

2.  Under  walls  of  houses  pieces  of  plank,  5  to  6  feet  long 
and  from  in.  to  3  in.  thick,  can  be  placed  side  by  side 
on  soft  materials,  thereby  securing  increased  bearing  sur¬ 
face,  and  by  using  from  two  to  four  courses  placed  at  right 
angles  to  each  other  the  base  can  be  spread  to  a  width  of 
IO  to  12  feet,  allowing  structures  of  considerable  weight  to  be 
built  on  very  soft  foundation-beds,  such  as  silt  or  quicksand. 
Sometimes  a  series  of  rough  logs  are  laid  longitudinally,  either 
side  by  side  or  at  short  intervals,  the  intervening  space  filled 
with  sand,  broken  stone,  or  concrete,  and  one  or  more  courses 
of  plank  placed  over  and  at  right  angles  to  these ;  or  two  or 
more  courses  of  logs  crossing  each  other,  the  intervening  spaces, 
if  any,  filled  as  stated  above.  This  last  constituted  the  founda¬ 
tion  of  the  New  Orleans  Custom-house,  a  large,  heavy,  and  mass¬ 
ive  granite  building  ;  it  is  true  that  in  this  some  settlement  has 
taken  place,  but  no  serious  damage  has  resulted.  By  either  of 
the  above  methods  many  houses,  culverts,  and  other  structures 
are  safely  and  economically  constructed.  In  such  cases  the 
probability  is  that  whatever  settlement  takes  place  will  be  uni¬ 
form  under  the  entire  structure,  and  no  damage  to  the  structure 
will  follow  unless  high  and  heavy  towers  or  steeples  are  bonded 
into  the  structure  ;  in  such  cases  the  spread  of  the  base  should 
be  such  as  to  insure  that  the  unit  pressure  shall  be  the  same  as 
under  any  other  portion  of  the  structure.  Piles  are  better  under 
such  very  heavy  loads,  and  should  always  be  used  if  there  is 
any  possibility  of  the  material  being  washed  or  scoured  out. 

3.  Under  large  and  heavy  piers  for  bridges  it  is  not  unusual 
to  build  cribs  made  of  round  logs  or  square  timber,  crossing 
each  other  and  bolted  together  at  each  intersection,  leaving 
cells  or  pockets  to  be  filled  with  broken  stone  or  concrete ;  in 
such  cases  the  crib  is  really  intended  to  confine  the  filling  ma¬ 
terial,  but  of  course  supporting  its  proportionate  share  of  the 
load.  If  the  filling  is  gravel  or  broken  stone,  iron  rods  should 
be  used  to  tie  the  sides  of  the  crib  together  so  as  to  prevent 


TIMBER  FOUNDATIONS. 


149 


any  tendency  to  bulging ;  this  is  not  necessary  when  concrete  is 
used.  The  dimensions  of  such  cribs  should  be  from  4  to  6  feet 
greater  all  around  than  the  masonry  structures  resting  upon 
them.  If  the  crib  has  to  be  sunk  through  any  depth  of  water,  a 
plank  bottom  will  have  to  be  used  over  the  entire  bottom,  or  at 
any  rate  under  a  sufficient  number  of  the  pockets  to  hold  the 
weight  necessary  to  overcome  the  buoyancy  of  the  water.  It  is 
not  advisable  to  endeavor  to  sink  the  cribs  by  building  the 
masonry,  as  the  cribs  will  rarely  rest  on  the  bottom  in  a  per¬ 
fectly  level  position.  With  broken  stone  or  concrete  filling  this 
is  a  matter  of  little  consequence,  unless  very  much  inclined, 
as  it  can  be  easily  levelled  with  broken  stone  or  concrete,  and 
the  masonry  commenced  by  the  use  of  a  cofferdam  if  neces¬ 
sary,  but  usually  the  crib  is  built  to  within  2  or  4  feet  of  the 
surface  of  the  water.  The  bed  of  the  stream  is  generally 
levelled  by  dredging;  this  serves  also  to  remove  the  soft 
and  loose  material  at  the  bottom,  but  it  generally  requires 
removing  the  material  over  a  large  surface,  if  it  is  desir¬ 
able  to  reach  any  great  depth  below  the  bed  of  the  river, 
adding  materially  to  the  cost ;  and  unless  stiff  clay  is  close  to 
the  bottom  the  obstruction  to  the  current  will  almost  always 
cause  a  scouring  action,  endangering  the  safety  of  the  structure. 
It  can  hardly  be  recommended  as  a  safe  and  satisfactory  foun¬ 
dation.  (See  Plate  IV,  Elevations,  Figs.  5  and  6.) 

4.  Many  examples,  however,  exist,  and  have  stood  the  test 
of  time.  The  Parkersburg  (W.  Va.)  bridge  across  the  Ohio 
River  was  thus  constructed ;  the  piers  of  this  bridge  stand  90 
feet  above  water,  and  rest  on  a  bed  of  gravel  and  sand  at  a 
depth  of  12  ft.  below  the  bed  of  the  river;  the  excavated  pit 
was  100  by  50  ft. ;  the  crib  or  grillage  was  composed  of  three 
courses  of  timber  12  in.  by  12  in.,  bolted  together,  78  ft.  long 
by  28  ft.  wide;  this  carried  a  pier  120  ft.  high,  with  spans  of 
350  ft.  Giving  a  pressure  of  9080  lbs.  or  \\  tons  (about)  to  the 
square  foot  on  gravel  and  sand.  A  rod  was  driven  25  ft.  into  this 
material.  An  open  caisson  was  built,  the  grillage  forming  the 
bottom,  this  was  sunk  on  the  gravel  bed  by  the  weight  of  the 
masonry  itself,  and  was  practically  level  when  it  rested  on  the 


150  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 

gravel.  These  piers  were  finished  15  ft.  wide  on  top;  10  ft. 
would  have  been  ample. 

5.  If  a  crib  is  to  be  sunk  on  a  bed  of  rock  which  is  very 
irregular  or  much  inclined,  two  methods  of  procedure  are 
open  : 

1st.  To  blast  the  rock  to  a  level  or  nearly  level  surface  ;  this 
is  difficult,  slow,  and  expensive. 

2d.  If  the  rock  is  irregular,  with  elevations  and  depressions, 
or  not  having  any  great  and  uniform  inclination,  the  crib  can  be 
sunk  until  it  almost  reaches  the  higher  point  or  points,  and 
while  suspended  in  this  position  broken  stone  can  be  dropped 
into  the  pockets  and  around  the  outside,  the  stone  assuming  its 
own  slope  below  the  crib,  and  this  continued  until  the  crib  is 
found  to  be  uniformly  and  solidly  supported  ;  the  masonry  can 
then  be  commenced.  Cement  can  be  forced  through  pipes  be¬ 
tween  the  broken  stone,  thereby  forming  a  solid  and  compact 
mass.  (See  Plate  IV,  Elevations,  Figs.  5  and  6.) 

'  COFFER-DAMS. 

6.  If  the  material  composing  the  bed  of  the  river  is  gravel, 
sand,  clay,  or  silt,  and  either  too  soft  to  build  upon,  or  is  at 
all  likely  to  be  scoured  out,  instead  of  the  preceding  methods, 
the  space  to  be  occupied  by  the  structure  must  be  sur¬ 
rounded  by  a  water-tight  dam  of  some  kind,  so  that  the 
water  can  be  pumped  out  of  the  enclosed  space,  and  the 
excavation  and  preparation  of  the  foundation-bed  proceeded 
with  as  on  dry  land.  The  structure  for  this  purpose,  of 
whatever  material  it  may  be  constructed,  is  called  a  coffer¬ 
dam.  If  there  is  no  material  depth  of  water,  not  exceed¬ 
ing  5  ft.,  and  no  current,  clay  either  alone  or  mixed  with 
sand  and  gravel  can  be  dumped  in  the  water,  so  as  to  form  an 
earthen  dam  entirely  around  the  space  to  be  enclosed,  and 
carried  up  2  or  3  feet  above  the  water  surface,  finished  at 
least  3  ft.  wide  on  top,  the  earth  assuming  its  own  slope  below 
the  water  surface  ;  this  slope  will  be  rather  flat,  from  two  to 
three  horizontal  to  one  vertical.  The  material  to  be  excavated, 


TIMBER  FOUNDATIONS. 


IS  I 

being  saturated  with  water  would,  also  require  a  long  flat 
slope,  consequently  the  area  enclosed  should  be  large  in  com¬ 
parison  with  the  area  of  the  base  of  the  structure  ;  for  instance, 
if  the  base  of  the  structure  is  to  be  20  ft.  wide  and  the  depth 
excavated  is  15  ft.  below  the  water  surface,  the  interior  width 
of  the  dam  should  not  be  less  than  from  80  to  no  ft.,  and  the 
length  generally  from  two  to  three  times  the  width.  Owing  to 
the  great  dimensions  required,  this  kind  of  dam  is  rarely  used, 
and  resort  is  had  to  the  ordinary  timber-coffer  dam,  constructed 
as  follows : 


Article  XXX. 

COFFER-DAMS  OF  TIMBER. 

7.  Two  rows  of  guide-piles,  the  piles  of  the  proper  length 
and  at  least  12  inches  in  diameter  at  the  larger  end,  are  driven 
entirely  around  the  space  to  be  enclosed.  The  area  of  this  space 
should  be  considerably  greater  than  the  largest  area  of  the 
structure  to  be  built.  If  the  dimensions  of  the  base  of  the 
structure  are  20  X  43  feet,  the  inside  dimensions  of  the  dam 
should  under  no  circumstances  be  less  than  6  feet  more  than 
the  above,  or  26  X  49  feet,  and  in  general  should  be  governed 
by  the  depth  to  be  excavated  below  the  bed  of  the  river,  the 
increase  being  at  least  equal  to  the  depth,  and  better  if  equal 
to  1^  times  the  depth;  or  for  a  depth  of  10  feet  below  the  bed 
of  the  river  the  dimension  in  the  above  case  should  not  be 
less  than  30  X  53  feet,  and  economy  will  justify  an  increase  to 
35  X  58  feet.  More  failures  in  coffer-dams  result  from  the 
fact  that  the  enclosed  area  is  made  too  small,  than  from  any 
other  cause.  A  false  idea  of  economy  in  the  beginning  gen¬ 
erally  results  in  much  loss  of  time  and  a  largely  increased 
expenditure  in  the  end.  The  dimension  of  the  dams  having 
been  settled,  the  two  rows  of  piles  are  driven  so  that  the 
piles  in  each  row  will  be  from  4  to  8  feet  apart,  and  the  rows  to 
be  from  5  to  8  feet  apart,  according  to  the  height  of  the  dam 
above  the  bed  of  the  river.  This  clear  distance  between  rows 


152 


A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


will  allow  from  3^  to  6%  feet  in  thickness  of  the  clay  puddle. 
This  width  is  required  to  give  stability  to  the  dam.  From  18 
inches  to  2  feet  of  good  clay  puddle  is  ample  to  prevent  leaks. 
Wale-pieces,  that  is,  horizontal  pieces  of  timber,  generally  6  X 
12  inches,  and  of  varying  lengths,  are  bolted  to  the  rows 
of  piles,  facing  each  other  between  the  rows.  Bolts,  1  inch 
diameter  and  from  7  to  9  feet  long,  tie  the  rows  together. 
These  are  placed  above  the  water  surface.  Another  set  of 
wales  should  now  be  placed,  resting  against  the  piles  at  or  near 
the  bed  of  the  river..  This  is  done  by  fastening  battens  to  a 
wale-piece  of  any  length,  and  forcing  the  wale-piece  to  the  bed 
of  the  river,  the  battens  then  spiked  at  their  upper  ends  to  the 
top  wale-pieces.  This  is  carried  all  around  both  rows  of  piles, 
leaving  spaces  or  gaps  between  the  ends  of  the  wales.  Then 
other  pieces  are  lowered  in  the  same  way,  resting  on  top  of  the 
first  pieces,  and  covering  the  vacant  spaces  between  them.  Inter¬ 
mediate  rows  of  wales  should  be  placed  as  above  described,  so 
that  the  vertical  distance  between  any  two  sets  of  wales  should 
not  exceed  6  feet.  Sheet-piles  (planks  of  about  2 J  to  3  inches 
thick,  and  of  lengths  depending  upon  the  depth  of  the  water) 
are  now  driven,  either  by  a  heavy  mall  or  a  light  hammer 
guided  by  leads  as  in  pile-drivers,  close  together  and  resting 
against  the  wales.  These  should  penetrate  from  18  inches  to 
5  or  6  feet  into  the  bed  of  the  river,  depending  upon  the  ma¬ 
terial  in  the  bed  of  the  river.  These  sheet-piles  are  sharpened 
at  the  lower  end,  so  that  the  bevel  extends  the  entire  width  of 
the  plank,  i.e.  from  7  to  12  inches,  and  when  driven  this  bevel 
tends  to  hold  each  plank  up  against  the  last  one  driven.  This 
forms  a  double  close  sheeting  entirely  around  the  enclosed 
space.  Each  plank  when  driven  is  spiked  to  the  upper  wale- 
piece.  This  then  leaves  a  space  to  be  filled  with  the  puddle, 
varying  from  3^  to  6|  feet  in  width.  The  guide-piles  should  be 
driven  well  into  the  bed  of  the  river,  and  if  practicable  should 
pass  through  any  sand  or  gravel  into  clay ;  but  if  the  dam  is 
made  large  enough,  a  penetration  of  from  10  to  15  feet  into 
the  bed,  of  whatever  material  that  may  be  found,  will  in  gen¬ 
eral  be  sufficient.  The  puddle  can  now  be  thrown  in  between 


TIMBER  FOUNDATIONS. 


153 


the  sheeting,  and  should  be  rammed  or  rather  cut  with  a  ram¬ 
mer  head  of  3-inch  plank  trimmed  to  a  wedge-shaped  edge. 
This  prevents  the  formation  of  distinct  layers.  Each  is  cut 
into  the  layer  below,  binding  the  entire  mass,  and  has  a  similar 
effect  to  the  ribbed  roller  used  in  making  reservoir  embank¬ 
ments.  The  dam  is  now  ready  to  be  pumped  out.  Many 
authorities  say  that  the  soft  and  loose  material  between  the 
sheeting  should  be  dredged  out.  The  writer  does  not  compre¬ 
hend  the  meaning  of  this.  It  is  not  necessary  if  the  bed  of  the 
river  is  clay,  nor  is  it  necessary  in  gravel  and  sand,  this  being 
considered  by  many  as  the  best  material  with  which  to  puddle. 
If  alluvial  soil  or  silt,  this  is  good  puddle  itself,  and  is  not  only 
water-tight,  but  often  air-tight.  It  can  hardly  be  necessary 
to  do  any  dredging,  unless  limbs  of  trees  or  brush  should  be 
encountered.  These  would  conduct  water  through  the  dam, 
and  might  cause  dangerous  leaks.  The  writer  at  least  never 
did  any  dredging  for  this  purpose,  and  has  had  good  success 
in  the  many  dams  constructed  by  him.  The  construction  of 
other  forms  of  dams  will  be  described  before  entering  into  a 
description  of  pumping  and  excavating,  as  these  process  will  be 
the  same  for  all.  (Figs.  3  and  4,  Plate  IV.) 

8.  Sometimes  four  rows  of  wales  are  used,  these  being 
placed  both  on  the  outside  and  inside  of  the  rows  of  piles,  and 
the  sheet-piles  are  driven  between  the  wale-pieces.  This 
guides  the  sheet-piles  to  some  extent  while  being  driven,  but 
has  no  other  advantage ;  requires  more  timber,  is  good 
practice,  though  not  necessary.  Sometimes  the  sheeting,  in¬ 
stead  of  being  3  inches  thick,  is  as  much  as  8  or  10  inches 
thick,  with  a  tongue  cut  on  one  face  about  2  to  2^  inches 
broad  and  about  the  same  depth,  and  a  similar  groove  cut  on 
the  opposite  face,  and  then  driven  so  that  the  tongue  of  one 
piece  fits  into  the  groove  of  the  adjacent  piece.  This  certainly 
causes  much  expense  in  framing,  and  also  delay  in  driving,  and 
great  waste  of  timber.  This  can  never  be  necessary  if  a 
double  wall  is  used,  but  will  make  a  good  single-wall  dam,  but 
will  require  strong  bracing  on  the  inside.  There  is  probably 
no  economy  in  this  plan.  Sometimes  a  groove  is  cut  on  both 


154 


A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


faces.  The  sheet-piles  are  then  driven  as  close  together  as 
possible,  and  a  2-inch  plank  is  driven  in  the  grooves.  This 
will  close  any  opening  that  may  exist  by  the  piles  leaning  from 
each  other  in  driving,  and  is  to  be  preferred  to  the  first  or 
tongue-and-grooved  method ;  or  strips  are  spiked  to  the  face  of 
the  piles,  one  strip  on  one  face  forming  a  tongue,  and  two 
strips  on  the  other  forming  a  groove,  and  driven  as  described. 
This  prevents  waste  of  timber,  but  is  not  as  good  as  either  of 
the  other  plans,  unless  the  timber  is  very  soft  or  splits  easily, 
in  which  case  the  strips  spiked  on  will  be  stronger  than  the 
regular  tongue  and  groove.  The  writer  thinks  that  the  ordi¬ 
nary  puddle-dam  will  prove  more  economical,  more  expedi¬ 
tious,  and  more  satisfactory  than  either  of  the  three  last-men¬ 
tioned  methods.  (See  Figs.  3A  and  4A,  Plate  IV.) 

9.  A  solid  wall  of  timber,  either  made  of  12  X  12  in.  sticks 
or  plank,  laid  horizontally  on  top  of  each  other  and  spiked  or 
bolted  together,  which  can  be  framed  floating  on  the  water, 
and  large  enough  to  enclose  the  required  area,  can  be  built 
and  sunk  into  a  dredged  hole,  or  resting  on  the  natural  bed  of 
the  river,  and  sheet-piles  driven  all  around  and  close  against 
the  timber  wall  to  which  they  are  spiked  or  bolted,  will  make 
a  good  dam,  but  will  have  to  be  strongly  braced  on  the  interior. 
This  dam,  when  built  in  a  circular  or  octagonal  form,  as  is 
required  in  case  of  pivot  piers  for  drawbridges,  has  many  ad¬ 
vantages,  is  easily  and  rapidly  constructed,  contains  a  minimum 
quantity  of  timber,  will  require  little  or  no  bracing  on  the 
interior,  and,  even  if  requiring  large  guide-piles  to  be  driven 
on  the  inside  to  stiffen  and  hold  it  steady,  will  prove  economi¬ 
cal.  This  will  make  a  good  dam,  when  the  bed  of  the  river  is 
a  rocky  ledge,  by  sinking  it  on  the  rock  and  throwing  clay 
puddle  all  around  on  the  outside,  unless  the  current  is  so  rapid 
as  to  wash  away  the  puddle.  In  this  case  a  double  wall  built 
as  above  described,  and  connected  by  cross-pieces  of  timbers 
for  strut  and  tie  braces,  dovetailed  or  bolted  to  the  walls, 
and  the  space  between  the  walls  filled  with  puddle,  will  make  a 
stronger  dam  than  any  other  described  ;  can  be  rectangular,  oc¬ 
tagonal,  or  circular  in  plan  ;  of  any  size  or  height  required  ;  will 


TIMBER  FOUNDATIONS. 


155 


need  little  or  no  interior  bracing.  Sheet-piles  should  be  driven 
in  earth  bottoms  as  deep  as  practicable,  and  on  rock  should  be 
driven  so  as  to  broom  or  batter  the  lower  ends,  so  that  they 
may  conform  to  the  irregularities  of  the  bottom  ;  this  will  hold 
the  puddle,  and  to  a  large  extent  prevent  leaks  along  the  rock 
under  the  puddle.  Where  the  rock  has  a  regular  inclination 
or  slope  this  crib-dam  can  be  easily  built  so  as  to  conform  to 
the  slope  of  the  bottom.  It  should  always  be  used  in 
case  of  a  rocky  bottom.  It  is  called  the  crib  coffer-dam.  In 
very  rapid  currents  it  can  be  built  in  sections  of  short  lengths 
shaped  as  truncated  wedges,  alternate  sections  held  and  sunk 
in  place  and  heavily  weighted,  the  closing  sections  then  floated 
and  forced  into  their  places  by  the  force  of  the  current,  and 
then  arrangements  for  holding  the  puddle  can  be  made  by 
uprights  and  sheeting  in  the  enclosed  space.  This  method 
can  be  used  where  the  above-described  method  scould  not  be 
used.  (See  Figs.  5  and  6,  Plate  IV.) 

10.  Mr.  J.  E.  Robinson  has  a  patent  dam  which  has  some 
merits  worthy  of  notice,  which  does  not,  however,  seem  capa¬ 
ble  of  economical  application,  except  in  shallow  water  and 
where  no  great  current  exists.  It  is  to  be  always  circular 
in  form,  regardless  of  the  shape  of  the  pier.  It  is  constructed 
as  follows:  A  series  of  shears  or  three  pieces  of  timber  held 
together  by  bolts  passing  loosely  through  the  pieces  at  the  top, 
allowing  the  legs  to  be  spread  out  at  any  angle  with  each 
other;  these  are  set  up  at  intervals  on  the  circumference  of  a 
circle ;  each  has  a  small  block  and  tackle  fastened  to  its  apex ; 
large  sheets  of  iron  plate  are  then  suspended  under  the  shears. 
These  plates  are  bolted  together,  forming  a  circular  sheeting ; 
inside  of  this  long  timbers,  12  in.  X  12  in.,  slightly  bevelled,  as 
in  arch  stones,  are  driven  with  a  light  hammer,  close  together 
and  resting  against  the  iron  sheeting.  The  space  thus  enclosed 
is  ready  then  to  be  pumped  out.  This  certainly  forms  a  strong 
dam,  even  without  interior  bracing,  when  properly  constructed, 
but  in  considerable  depths  radial  bracing  will  be  found  neces¬ 
sary  to  prevent  the  bottom  of  the  timbers  from  pressing  in¬ 
wards.  The  objections  or  defects  arise  mainly  from  driving 


1 56  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 

the  piles,  as  it  is  difficult  to  keep  them  in  contact  for  their  full 
length,  and  they  will  almost  always  separate  or  spread  at  their 
lower  ends ;  this  can,  at  least  in  part,  be  overcome  by  cutting 
grooves  in  the  piles  and  driving  filling  pieces  of  iron  or  wood 
in  the  grooves,  as  previously  described.  Owing  to  the  form  of 
the  dam,  the  excavation  is  confined  to  a  narrow  area  in  the 
direction  of  the  length  of  the  pier,  coming  close  to  the  sides  of 
the  dam  along  one  diameter,  and  leaving  broad,  unexcavated 
spaces  on  either  side.  In  addition,  the  excavated  material  is 
for  convenience  thrown  on  top  of  the  undisturbed  earth  ;  the 
tendency  of  this  is  to  bulge  the  sides  out,  and  the  great  excess 
of  outside  pressure,  in  a  direction  at  right  angles  to  this,  tends 
to  force  this  part  of  the  dam  in,  and  aids  in  forcing  the  other 
parts  outwards  This  distorts  the  entire  dam  ;  a  portion  of  the 
dam  comes  in  and  another  portion  goes  out,  causing  leaks  that 
are  hard  to  be  controlled.  In  the  writer’s  experience  these 
dams  gave  great  trouble  and  caused  much  delay ;  but  in  one 
case,  at  the  Schuylkill  River,  this  was  used  where  an  ordinary 
coffer-dam  had  failed  entirely,  and  under  circumstances  pecu¬ 
liarly  trying  to  a  coffer-dam.  This  will  be  alluded  to  in  another 
paragraph. 

II.  In  whatever  manner  the  coffer-dam  may  be  constructed, 
with  or  without  the  puddle,  the  first  step  is  to  pump  out 
the  water  This  can  be  done  by  any  of  the  ordinary  pumps, 
such  as  the  force,  lift,  or  centrifugal  pump.  A  single  pump 
discharging  a  stream  of  from  4  to  10  inches  in  diameter  should 
always  be  sufficient  to  keep  the  water  out  of  a  coffer-dam,  and 
will  ordinarily  prove  sufficient  ;  but  if  the  dam  is  badly  con¬ 
structed,  or  unexpected  and  large  leaks  are  developed,  it  may 
require  several  pumps.  There  are  two  forms  of  the  centrifugal 
pump  :  one  in  which  the  vanes  are  in  a  horizontal  casing  which 
is  placed  at  the  bottom  of  the  excavation,  and  is  lowered  as  the 
excavation  proceeds,  the  discharge  connected  with  this,  and 
lengthened  as  required  ;  this  forces  the  water  upwards  and  dis¬ 
charges  it  over  the  top  of  the  dam  ;  in  the  other  the  vanes  are 
in  a  vertical  casing,  and  is  generally  placed  on  or  near  the  top 
of  the  dam  ;  a  pipe  extends  to  the  bottom  of  the  excavation  and 


TIMBER  FOUNDATIONS. 


157 


is  lengthened  as  required  ;  this  lifts  the  water  to  the  top  and  dis¬ 
charges  it  over  the  dam.  Either  of  these  pumps  will  throw  a 
6  or  10  inch  stream,  and  should  be  ample  for  any  dam  properly 
constructed.  The  force-pump  is  placed  either  on  top  of  the  dam, 
or  at  the  bottom  of  the  excavation,  or  at  any  intermediate  point, 
and  will  throw  any  diameter  of  stream  required.  The  force- 
pump  is  apt  to  give  more  or  less  trouble  by  the  accumulation  of 
small  fibres  of  wood,  leaves,  or  small  gravel  and  sand  in  the  valves 
and  between  the  sliding  plates,  causing  delay  and  frequent  stop¬ 
ping  of  the  pump,  often  at  a  critical  period  of  the  work,  no  mat¬ 
ter  how  carefully  the  suction  end  of  the  pipe  may  be  protected  by 
screens  and  strainers.  In  all  important  works  duplicate  pumps 
should  be  on  hand,  unless  they  can  be  obtained  without  delay, 
as  the  stoppage  of  a  work  of  this  kind  might  cause  a  suspen¬ 
sion  of  the  work  for  a  season,  or  the  breaking  and  loss  of  a 
coffer-dam.  A  centrifugal  pump  is  less  liable  to  get  out  of 
order,  as  it  will  readily  discharge  small  chips,  sand,  and  grit 
without  damage  or  stoppage  to  the  working  of  the  pump. 
(Plate  V,  Figs.  I,  2,  3,  and  4.) 

12.  When  the  water  is  pumped  out,  and  no  serious  leaks 
have  developed,  the  excavation  of  the  bottom  can  be  com¬ 
menced.  The  material,  as  far  as  practicable,  should  be  kept 
piled  up  against  the  sides  of  the  dam,  and  if  the  proper  area 
has  been  enclosed  there  will  be  little  danger  of  undermining 
the  dam,  or  of  forcing  in  the  sides  of  the  dam  by  the  outside 
pressure,  when  the  distance  from  the  water-level  to  the  bot¬ 
tom  of  the  excavation  does  not  exceed  25  feet,  and  this  with¬ 
out  interior  bracing,  in  a  double-wall  puddle-dam.  Braces 
should  always  be  omitted,  if  possible  (but  in  all  cases  timber 
should  be  kept  in  convenient  positions,  and  of  proper  lengths, 
so  that  they  can  be  readily  and  rapidly  used  if  any  signs  of 
yielding  are  observed),  as  they  materially  interfere  with  and 
delay  the  construction  of  the  masonry.  A  few  braces  placed 
diagonally  across  the  corners  of  a  rectangular-shaped  dam  will 
add  greatly  to  the  strength  of  the  dam,  and  will  be  practically 
no  obstruction  to  the  work.  It  will  be  found  advisable,  both 
to  limit  the  amount  of  excavation,  increase  the  space  on  which 


158  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 

the  excavated  material  can  be  deposited  on  the  inside  of  dam, 
and  at  the  same  time  increase  the  height  to  which  it  may 
be  piled  against  the  side  of  the  dam,  to  place  a  row  of 
timber  around  the  area  to  be  occupied  by  the  masonry,  leav¬ 
ing  a  small  margin  all  around,  and  then  to  drive  sheeting  be¬ 
hind  this.  As  the  excavation  proceeds  drive  the  sheeting 
farther  down.  In  a  large  coffer-dam  in  the  Ohio  River  the 
writer  placed  a  double  row  of  12  X  12  inch  timbers  around  the 
area  required  to  be  excavated,  as  soon  as  the  water  was 
pumped  out.  On  the  outside  of  this  a  3-inch  plank  was  bolted 
so  as  to  leave  a  space  of  about  4  inches  between  the  timber 
and  the  plank.  Sheeting  plank  from  6  to  8  feet  long  was 
then  driven  two  or  three  feet  into  the  bed  of  the  river.  This 
provided  a  space  of  about  6  feet  all  around  upon  which  the 
excavated  material  could  be  deposited.  As  the  excavation  ad¬ 
vanced  a  man  was  kept  driving  the  sheeting  down.  The  plank 
bolted  to  the  square  timber  prevented  the  upper  end  of  the 
sheeting  from  inclining  outward,  and  consequently  the  lower 
ends  from  pressing  inwards,  and  the  excavation  continued 
through  about  8  feet  of  gravel  and  sand  to  the  clay.  By  this 
simple  arrangement  the  excavation  was  confined  to  the  exact 
area  required.  The  material  was  simply  cast  by  the  shovel 
against  the  sides  of  the  dam,  both  of  which  reduced  the  cost 
greatly,  and  the  dam  was  well  supported  from  the  inside. 
Leaks  were  almost  entirely  prevented,  and  not  a  brace  was 
used  from  the  beginning  to  the  end.  The  depth  from  the 
water-level  to  the  bottom  of  the  excavation  was  about  23  feet, 
the  length  of  this  coffer-dam  on  the  inside  60  feet,  and  width 
34  feet.  The  thickness  of  the  sides  of  the  dam  was  intended 
to  be  8  feet,  but,  owing  to  careless  driving  of  piles,  it  was  not 
more  than  6  feet  in  places.  The  thickness  of  the  puddle  itself 
varied  from  2%  to  5  feet. 

13.  In  another  dam  somewhat  carelessly  constructed,  and 
which  showed  evident  signs  of  weakness,  of  about  the  same 
•size  as  the  above,  but  requiring  a  wider  and  longer  pier,  an 
inner  crib  was  constructed,  while  the  dam  was  filled  with  water, 
as  follows  :  Horizontal  timbers  were  framed  together  so  as  to 


TIMBER  FOUNDATIONS. 


159 


enclose  a  space  somewhat  larger  than  the  bottom  area  of  the 
masonry.  The  bottom  layer  was  cut  diagonally,  so  as  to  form 
a  cutting  edge.  Another  12  X  12  inch  layer  was  bolted  to  this. 
3-inch  plank  8  feet  long  was  then  spiked  on.  At  the  corners 
and  at  intervals  on  the  sides  and  ends,  posts  12X12  inches  X 
4  feet  were  placed  vertically  on  the  horizontal  pieces,  to 
which  the  plank  was  spiked.  On  the  posts  another  course  of 
horizontal  timbers  was  placed,  and  so  on  until  the  crib  rested 
on  the  bottom.  The  space  between  the  crib  and  coffer-dam 
was  then  filled  with  earth.  The  water  was  then  pumped  out. 
A  few  braces  were  placed  on  the  inside  of  the  crib  ;  the  ex¬ 
cavation  was  then  commenced,  the  crib,  weighted  with  large 
stone,  settled  gradually.  After  excavating  a  few  feet,  the 
coffer-dam  commenced  to  be  undermined  ;  the  material  between 
crib  and  coffer-dam  commenced  to  flow  to  the  interior.  The 
sides  of  the  dam  bulged  inwards  until  they  rested  against  the 
crib.  The  puddle  in  the  coffer-dam  settled ;  the  pumps  could 
not  keep  down  the  water.  It  was  decided  to  flood  the  dam  for 
fear  that  the  crib-dam  and  braces  could  not  stand  the  pressure. 
After  carefully  considering  the  conditions,  two  plans  presented 
themselves.  One  was  to  build  a  new  dam  ;  this  would  take  a 
long  time,  cost  a  great  deal  of  money.  The  other  was  to  re¬ 
puddle  the  old  dam,  and  also  to  throw  a  large  quantity  of 
puddle  around  the  old  dam  on  the  outside,  pump  the  water 
out,  and  then  brace  strongly  the  crib,  and  endeavor  to  reach  a 
safe  foundation,  the  inner  crib  holding  the  coffer-dam  in  place. 
This  was  the  most  expeditious,  and  seemed  practicable,  if  we 
could  pump  the  water.  This  was  decided  upon  and  acted 
upon  at  once.  The  water  was  again  pumped  out.  Bracing 
the  crib  strongly  as  the  water  fell.  The  dam  rested  hard  against 
the  crib  in  places,  creating  great  frictional  resistance,  requiring 
a  largely  increased  weight  to  sink  the  crib.  The  excavated 
material  was  placed  between  the  crib  and  the  dam  ;  it  would 
continually  flow  back.  Such  material  as  was  not  placed  be¬ 
tween  the  crib  and  dam  was  lifted  out  in  buckets  and  dumped 
into  the  river.  In  this  manner,  with  much  delay,  and  with 
slow  progress,  we  succeeded  in  reaching  the  clay  upon  which 


160  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 

the  structure  was  built,  after  excavating  through  14  feet  of 
gravel  and  sand.  This  inner  crib  has  the  following  advan¬ 
tages  :  1.  It  reduces  the  amount  of  material  to  be  excavated  to 
a  minimum,  saving  time  and  money.  2.  It  enables  us  to  keep 
the  lower  part  of  the  main  and  sheet  piles  always  covered,  pre¬ 
venting  undermining  of  the  dam,  and  great  inflow  of  water, 
sand,  and  gravel,  by  keeping  the  space  between  the  dam  and 
crib  always  filled  with  material.  3.  The  crib  can  be  sunk  even 
below  the  points  of  the  main  or  guide  piles  without  danger. 
The  space  between  the  crib  and  dam  should  not  be  less  than 
5  or  6  feet.  Great  depths  can  be  reached  by  this  means,  and 
the  crib  should  always  be  used  if  the  depth  is  over  20  to  25 
feet.  The  crib  can  be  built  up  as  the  excavation  and  sinking 
progresses.  (See  Figs.  1,  2,  3,  and  4,  Plate  III.) 

14.  In  many  cases  a  crib  of  this  kind  can  be  used  without 
any  puddle-dam  on  the  outside.  This  would  require  the  plank 
sides  to  be  calked  to  prevent  too  great  an  inflow  of  water. 
A  modification  of  this  construction  is  used  as  a  coffer-dam,  for 
the  sides  of  the  open  caisson,  or  on  top  of  a  pneumatic  caisson, 
and  of  heights  from  20  to  40  ft.  This  will  hereafter  be  ex¬ 
plained  and  illustrated. 

15.  The  puddle  filling  in  coffer-dams  is  generally  composed 
of  such  earth  as  is  easily  accessible.  Some  materials  are  better 
than  others,  but  any  ordinary  clay  or  loam  will  make  good 
puddle  if  the  layers  are  well  bonded  or  cut  into  each  other. 
What  is  known  as  brick-clay  makes  an  excellent  puddle.  Some 
high  authorities  recommend  a  sand  and  gravel  filling,  which  is 
claimed  to  be  better  than  clay ;  and  should  a  large  leak  occur, 
the  sand  and  gravel  will  fall  and  fill  the  cavity,  and  can  be  re¬ 
filled  on  top.  This  is  true  ;  but  water  will  always  find  its  way 
through  sand  and  gravel,  where,  if  well  mixed  with  clay,  or  clay 
alone,  it  is  practically  impervious  to  water.  The  writer  pre¬ 
fers  greatly  this  last  material. 

16.  Coffer-dams  generally  prove  to  be  expensive,  are  always 
uncertain,  and,  unless  sufficiently  large,  or  some  form  of  inner 
support  used  as  above  described,  will  give  trouble,  frequently 


TIMBER  FOUNDATIONS. 


161 


filling  up  with  water  and  earthy  material,  or  undermining,  and 
not  unfrequently  breaking  in. 

17.  Sometimes  coffer-dams  are  used  when  it  is  intended  to 
drive  piles  in  the  enclosed  space  for  the  foundations,  no  ma¬ 
terial  being  removed  from  the  bed  of  the  river,  but  simply  to 
keep  the  water  out  while  the  piles  are  being  cut  off  and  a 
timber  platform  framed  on  top.  This  platform  consists  of 
several  courses  of  square  timber.  The  piles  being  cut  off  at  the 
same  level,  they  are  then  capped  by  12  X  12  in.  timber,  then 
another  course  at  shorter  intervals  placed  across  and  at  right 
angles  to  the  caps,  and  over  this  another  course  of  square  tim¬ 
ber  placed  close  together,  forming  a  solid  timber  floor,  the 
whole  thoroughly  bolted  together  with  drift-bolts.  The  open 
spaces  are  sometimes  filled  with  broken  stone  or  concrete, 
around  the  tops  of  the  piles,  between  and  under  the  timbers; 
this  is  not  necessary  unless  the  material  is  very  soft  or  yielding, 
and  is  only  intended  in  this  case  to  give  lateral  stiffness  and 
steadiness  to  the  piles  ;  or  the  timber  platform  may  be  omitted, 
and  a  thick  bed  of  concrete  placed  around  and  over  the  top  of 
the  piles. 

18.  If  the  material  has  been  removed  to  the  proper  depth, 
the  bed  of  gravel,  sand,  or  clay  is  levelled.  The  masonry  may 
be  commenced  directly  on  these  materials,  but  it  is  better  to 
first  lay  a  bed  of  concrete  not  less  than  2  to  3  feet  in  thickness 
over  the  area,  and  extending  a  distance  equal  to  the  thickness, 
outside  of  the  space  to  be  occupied  by  the  masonry  ;  or  a  course 
of  square  timber  may  be  laid  on  the  foundation-bed  ;  or  the 
two  may  be  combined,  timber  being  embedded  in  the  concrete. 

19.  Coffer-dams  are  rarely  used  where  the  material  has  to  be 
excavated  to  any  great  depth,  for  the  reasons  above  described. 
Some  other  method  will  be  in  general  preferred,  such  as  the 
open  caisson. 


A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


162 


Article  XXXI. 

OPEN  CAISSON. 

20.  An  open  caisson  is  simply  a  water-tight  box  open  at 
the  upper  end,  and  is  constructed  as  follows:  A  floor  of  square 
timber,  12  X  12  in.,  built  of  two  or  more  courses  of  timber, 
well  bolted  together,  and  of  any  size  and  shape,  but  from 
4  to  6  ft.  larger  than  the  proposed  structure;  this  floor  is  to 
be  thoroughly  calked.  Near  the  outer  edges  of  this,  large 
bolts  with  eyes  are  fastened  ;  about  one  foot  inside  of  the  bolts 
square  timber  sills  12  X  1 2  in.  are  placed  ;  vertical  posts  12  X  12 
in.  are  connected  to  these  by  mortise-and-tenon  joints.  On 
the  top  of  these  posts,  which  are  placed  from  4  to  5  ft.  apart, 
caps  of  12  X  12  in.  timber  are  placed  and  secured;  these 
skeleton  frames  are  then  covered  by  two  courses  of  sheeting 
plank,  the  inner  course  placed  diagonally,  the  outer  course 
horizontally;  over  the  top  of  the  sides  12  X  12  in.  pieces  are 
placed,  projecting  over  the  sides  and  ends  not  over  1  to  i^-  ft. 
Long  iron  rods  with  hook  at  one  end  and  screw-threads  at  the 
other  are  hooked  to  the  eye-bolts  and  passed  through  holes  in 
the  top  pieces ;  a  washer  is  placed  on  top,  the  nut  being 
screwed  on  so  as  to  bring  the  sides  to  a  hard  bearing  on  the 
floor  or  platform.  These  sides  can  be  of  any  height,  but  it  is 
best  to  use  a  height  of  not  more  than  15  to  20  ft.  at  first,  and 
when  sunk  to  this  depth  add  another  section  similarly  con¬ 
structed  on  top.  I11  this  case  a  thimble  or  sleeve  with  right-  and 
left-handed  threads  should  be  used  instead  of  the  nut  on  the 
end  of  the  rod,  so  that  other  sections  of  rods  can  be  connected 
as  the  sections  of  the  sides  are  added.  As  soon  as  the  sides  are 
brought  to  a  firm  bearing  on  the  floor  the  outside  planking 
should  be  thoroughly  calked. 

21.  The  caisson  is  now  ready  to  be  floated  to  the  site  of 
the  structure.  A  course  or  two  of  masonry  should  be  laid  to 
steady  the  caisson  and  prevent  any  tendency  to  careen  or  turn 


OPEN  CAISSON. 


163 


sideways.  If  the  bed  of  the  river  is  of  a  firm  material — clay, 
sand,  or  gravel — it  is  only  necessary  to  level  the  bed.  If  there 
is  danger  of  scouring  when  the  current  is  strong,  or  the  ma¬ 
terial  is  too  soft  to  bear  the  load,  piles  must  be  driven  and 
then  cut  off  to  a  level  either  at  the  bed  of  the  river  or  at  some 
point  below  the  water  surface.  In  either  case  the  building  of 
the  masonry  is  continued  until  the  caisson  nearly  reaches  the 
bed  of  the  river  or  the  top  of  the  piles,  as  the  case  may 
be.  The  caisson  carefully  adjusted  or  located  in  its  exact 
position  by  ropes  attached  to  anchors  or  to  guide-piles  driven 
for  the  purpose,  enough  water  is  now  let  in  to  complete  the 
sinking.  If  it  does  not  rest  in  a  level  and  easy  position,  or 
veers  out  of  place,  the  water  is  pumped  out,  the  caisson  lifts, 
the  bed  or  piles  properly  levelled,  and  the  caisson  again  sunk ; 
and  this  must  be  repeated  until  the  result  is  satisfactory.  In 
the  case  of  resting  on  piles,  a  diver  should  be  sent  down  to  see 
that  it  rests  practically  on  all  the  piles,  as  with  a  light  load  it 
might  be  supported  by  only  a  few  piles,  and  the  increased 
weight  would  cause  the  piles  to  sink  ;  but  even  in  this  event 
the  only  effect  would  be  to  settle  until  other  piles  were  brought 
into  a  bearing,  and  any  increase  of  height  of  structure  can  be 
supplied  by  the  courses  of  masonry.  Any  number  of  piles  can 
be  sawed  off  under  water  so  as  not  to  vary  in  level  more  than 
one-quarter  inch  ;  this  difference  can  cause  no  harm.  The 
structure  can  then  be  completed.  When  this  rises  above  the 
water  surface  the  iron  rods  can  be  unhooked  and  removed  ;  the 
sides  of  the  dams  can  then  be  lifted  and  used  on  other  piers.  To 
enable  this  to  be  done  the  corner  posts  are  made  in  two  pieces 
and  held  together  by  a  pin-bolt,  which  can  be  removed,  and 
the  sides  and  ends  easily  fall  apart.  The  sides  of  the  dams 
should  be  braced  against  the  masonry  by  short  blocks  as  the 
caisson  sinks.  It  will  hardly  prove  economical  to  use  the  sides 
over  again,  unless  timber  is  scarce  or  costly.  The  manner  of 
cutting  piles  off  under  water  will  be  explained  under  head  of 
Piles  (Plate  IV,  Figs.  1  and  2,  also  Plate  V,  Figs.  1 A  and  2 A). 

22.  In  depths  of  water  not  exceeding  20  to  25  feet  this 
method  of  construction  is  simple  and  economical.  The  bottom 


164  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


of  the  caisson  forms  a  part  of  the  permanent  structure.  The 
masonry  is  being  built  up  as  the  caisson  sinks,  which  is  not 
the  case  in  a  puddle  coffer-dam.  The  material  can  be  cheaply 
excavated  and  levelled  by  dredging,  providing  that  a  dredging 
machine  can  be  hired  at  a  reasonable  cost,  or  enough  of  it  is. 
to  be  done  to  justify  a  contractor  to  undertake  the  work. 
These  sides  are  properly  a  single-wall  coffer-dam,  calking 
being  substituted  for  the  puddle,  and  interior  bracing  used 
instead  of  thickness  of  dam  to  secure  strength  and  stability. 


Article  XXXII. 

CUSHING  CYLINDER  PIERS. 

23.  THESE  piers  are  constructed  as  follows:  A  cluster  of 
piles  is  driven,  to  a  solid  bearing  or  to  a  satisfactory  resistance, 
as  close  together  as  practicable,  and  in  juxtaposition,*  if  it  can 
be  done;  each  cluster  is  composed  of  from  4  to  12  piles, 
bolted  together  strongly  at  or  near  their  upper  ends.  The 
piles  should  be  straight  and  of  good  size,  not  less  than  12 
inches  square  or  diameter  at  large,  or  less  than  9  or  10  inches 
at  smaller  end.  In  driving  such  clusters  the  pile  should  be 
slightly  inclined  or  raking  while  being  driven,  and  the  tops 
subsequently  pulled  together  and  held  by  bolts.  The  piles 
should  penetrate  the  soil  not  less  than  20  feet,  and  as  much 
more  depending  on  the  nature  of  the  material  as  to  softness  or 
hardness,  and  should  reach  well  below  any  danger  from  scour¬ 
ing  action.  Iron  cylinders  from  4  to  10  feet  in  diameter 
should  then  be  sunk  around  the  piles  at  least  to  a  depth  of  10 
feet  below  the  bed  of  the  river.  These  cylinders  are  made  in 
sections  of  from  5  to  10  feet  in  length  and  of  a  thickness  from 
1  to  1^  inches  of  metal,  with  internal  flanges  about  3  inches  wide. 
The  lower  edge  of  the  bottom  section  should  be  brought  to  a 
feather  or  cutting  edge.  Cast-iron  is  generally  used.  Enough 

*The  same  number  of  piles,  if  driven  at  small  intervals  apart,  would  carry 
greater  loads,  as  a  larger  aggregate  area  of  surface  would  be  available  for  fric¬ 
tional  resistance,  but  require  larger  cylinders. 


CUSHING  CYLINDER  PIERS.  1 65 

of  these  sections  are  bolted  together  to  reach  from  the  bed  of 
the  river  to  a  point  above  the  water  surface.  This  being 
placed  around  the  pile  clusters,  another  section  is  bolted  on. 
The  material  is  then  removed  from  the  inside  or  stirred  and 
loosened  so  that  the  cylinder  will  either  sink  by  its  own  weight 
or  by  adding  weights  on  top.  When  it  reaches  the  proper 
depth,  the  interior  of  the  cylinder  is  filled  with  concrete.  The 
water  may  be  pumped  out  if  the  material  at  the  bottom  is 
clay  or  silt,  and  in  some  cases  if  of  compact  sand ;  but  com¬ 
monly  the  water  remains  and  the  concrete  is  simply  thrown 
in.  When  filled  a  thick  iron  cap  from  2-J-  to  3  inches  thick  is 
bolted  to  the  top.  For  that  portion  of  the  cylinder  above  the 
bed  of  the  river  the  flanges  can  be  cast  on  the  outside.  Two 
of  such  cylinders  and  pile  dusters,  bolted  and  braced  to  each 
other  at  the  proper  distance  apart,  constitute  a  pier.  Four 
may  be  used  to  a  pier,  which  greatly  increases  the  stability. 
A  two-cylinder  pier,  if  it  stands  any  great  height  above  the 
bed  of  the  river,  is  wanting  in  lateral  stability  ;  and  if  the  ob¬ 
struction  caused  by  the  piers  causes  any  scouring,  the  safety 
of  the  structure  is  greatly  endangered,  calls  for  frequent  ex¬ 
amination,  and  requires  for  security  the  constant  and  liberal 
use  of  riprap  or  broken  stone  around  the  cylinders.  The  piles 
should  be  cut  off  at  or  below  low  water.  In  this  form  of  pier 
the  piles  are  the  real  supporting  power.  The  concrete  carries 
the  load  from  the  structure  above  to  the  piles;  the  cylinders 
merely  serve  as  a  casing  to  hold  the  concrete.  If  in  sand  or 
gravel,  the  friction  on  the  sides  of  the  cylinder  will  give  some 
additional  support.  These  piers  are  economical,  easily  and 
rapidly  constructed,  but  are  wanting  in  stability.  (See  Plate 
IV,  Figs.  7  and  8.) 

24.  In  many  cases,  both  for  railroad  and  highway  bridges, 
the  piles  are  omitted  ;  the  cylinders  are  sunk  into  the  material 
anywhere  from  3  to  over  100  feet,  and  are  then  filled  with  con¬ 
crete.  In  such  cases  the  cylinders  are  of  large  size  for  railroad 
bridges,  are  generally  larger  at  the  bottom,  and  taper  or  batter 
to  the  top.  They  are  capable  of  bearing  heavy  vertical  loads, 
are  wanting  in  stability,  especially  if  subjected  to  heavy  lateral 


1 66 


A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


pressures  resulting  from  moving  ice  and  drift  in  large  masses, 
or  exposed  to  great  and  repeated  shocks.  Their  cheapness 
has  led  to  a  frequent  use  for  highway  bridges  and  compara- 
atively  short  spans.  Some  examples  will  be  given  in  another 
paragraph  of  such  piers,  with  methods  of  sinking,  dimensions, 
heights,  etc. 


Article  XXXIII. 

SOUNDINGS  OR  BORINGS. 

25.  The  importance  of  a  thorough  knowledge  of  the  mate¬ 
rial  underlying  the  site  of  any  proposed  bridge  is  so  evident 
and  great  that  it  would  seem  unnecessary  to  more  than  allude 
to  the  subject ;  but  the  fact  is  that  engineers,  from  a  notion  of 
false  economy,  neglect  to  gain  the  necessary  information,  and 
as  a  result  design  and  prosecute  the  construction  of  even  im¬ 
portant  structures  either  ignorant  or  at  least  with  but  very 
meagre  knowledge  of  the  nature  and  lay  of  the  strata,  and 
when  too  late  find  themselves  involved  in  difficulties  which 
will  cost  thousands  of  dollars  to  overcome,  which  could  be 
saved  by  the  expenditure  of  not  over  two  to  three  hundred 
dollars,  expended  in  judicious  and  thorough  soundings,  to  say 
nothing  of  the  delay  and  stoppage  of  the  work.  Thorough 
sounding  will  always  be  money  well  spent  and  time  saved. 
For  these  reasons  this  subject  will  be  explained  in  some  detail. 

25l  The  first  and  usual  method  is  simply  to  drive  an  iron- 
rod  or  pipe  from  1  to  i|-  inches  diameter.  A  rod  of  this  kind 
can  sometimes  be  driven  to  a  depth  from  10  to  30  feet  by  con¬ 
stant  hammering  and  turning  after  each  blow.  The  informa¬ 
tion,  however,  obtained  is  meagre  and  unsatisfactory;  the 
character,  thickness,  or  lay  of  the  different  strata  penetrated 
cannot  be  determined  ;  boulders,  even  small  ones,  will  stop 
the  driving,  as  will  also  logs  or  drift  or  wrecks.  Reports 
based  upon  such  soundings  lead  to  erroneous  conclusions ; 
designs,  estimates,  and  contracts  are  made  that  require  change 
and  alterations,  involving  often  largely  increased  cost  and 
delays.  In  one  important  bridge  across  the  Ohio  River,  plans. 


SOUNDINGS  OR  BORINGS. 


167 

estimates,  and  contracts  were  closed  on  the  supposition  that 
rock  would  be  reached  at  a  short  depth,  and  this  information 
was  apparently  confirmed  by  the  fact  that  another  bridge  in 
sight  was  built  on  rock  at  a  short  distance  below  the  water 
surface ;  but  subsequently  it  was  discovered  that  no  firm 
material  existed  under  from  60  to  70  feet  below  low  water,  and 
the  foundations  had  to  be  constructed  by  the  pneumatic 
process.  Every  engineer  knows  the  cost  of  such  a  change  in 
the  conditions  and  terms  of  a  contract. 

26.  A  better  and  more  satisfactory  method  is  to  sink  a  terra¬ 
cotta  or  iron  pipe  from  3  to  8  in.  in  diameter,  as  follows :  The 
pipe  is  pressed  into  the  surface  as  far  as  practicable  ;  then  a 
long  narrow  bucket  with  a  cutting  edge,  and  a  flap  valve  a  little 
distance  above  the  cutting  edge  opening  upwards,  is  dropped 
into  the  pipe  and  is  alternately  and  rapidly  raised  and  dropped. 
The  material  will  be  collected  in  the  bucket,  and  at  intervals 
the  bucket  is  lifted  entirely  out  and  the  contents  of  the  bucket 
are  emptied.  This  is  repeated  ;  the  pipe  will  gradually  sink,  a 
man  standing  on  top  if  it  is  necessary.  Other  sections  of  pipes 
are  added  from  time  to  time.  It  will  be  necessary  sometimes 
to  pour  water  into  the  pipe  to  aid  in  the  cutting  and  flow  of 
the  material  into  the  bucket.  The  bucket  should  be  connected 
by  a  rope  passing  over  a  sheave  connected  with  a  frame  or 
shears  above.  Great  depths  can  be  reached  by  this  method 
with  reasonable  rapidity,  and  at  no  great  cost.  The  writer 
used  this  on  the  Ohio  River  with  satisfactory  results.  It 
enables  you  to  determine  the  thickness  and  nature  of  the 
strata;  that  is,  you  can  determine  whether  the  strata  is  sand 
or  gravel  or  clay,  but  you  do  not  know  in  what  condition  it 
exists.  Clay  will  be  brought  up  in  the  condition  of  mud.  The 
strata  may  contain  a  large  quantity  of  water,  which  would 
render  it  unsuitable  for  a  foundation-bed  when  the  overlying 
strata  are  removed.  An  experienced  well-borer  can  form  a 
good  opinion  on  these  matters.  On  the  whole  much  valuable 
and  satisfactory  information  can  be  gained,  and  it  is  greatly  to 
be  preferred  to  the  rod  process.  The  rod  will  often  bend  above 


1 68  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 

or  below  the  ground,  and  this  often  misleads,  as  the  bending 
apparently  renders  the  depth  actually  penetrated  uncertain. 

27.  The  third  method  is  simple,  relatively  inexpensive,  and 
entirely  satisfactory.  All  that  is  needed  is  a  number  of  sections 
of  pipe,  i^-  in.  diameter  and  f  in.  diameter,  a  small  hand  force- 
pump,  and  a  small  barge  or  raft.  Threads  are  cut  on  both 
ends  of  every  section  of  pipe,  and  a  number  of  thimbles  with 
internal  screw-threads  to  fit.  A  chisel-steel  wedge-shaped 
section  about  1  ft.  long,  with  two  small  holes  passing  through 
the  faces  of  the  wedge  into  a  hollow  tube,  made  to  fit  the  f-in. 
pipe.  An  upper  section  also  about  a  foot  long  has  a  solid 
handle  at  right  angles ;  a  short  hollow  elbow  is  connected  with 
this,  to  which  a  hose  from  the  pump  can  be  connected. 
Enough  sections  of  the  i^-in.  pipe  are  connected  together  to 
reach  from  the  surface  of  the  water  to  the  bed  of  the  river. 
This  pipe  is  lowered  to  the  bottom  and  pressed  4  or  5  ft.  in 
the  bottom  ;  the  top  is  then  secured  to  the  boat  or  raft.  When 
the  water  is  deep  and  the  current  is  rapid,  this  pipe  is  liable  to 
bend  considerably,  and  might  part  at  some  joint.  A  rope  at¬ 
tached  to  a  small  anchor  and  to  the  pipe  at  some  point  below 
the  water  will  relieve  this,  the  anchor  being  dropped  well  up 
stream.  The  f-in.  pipe  is  now  connected  together  with  the 
chisel  end  at  the  bottom  and  lowered  in  the  i^-in.  pipe;  the 
hose  then  connects  the  smaller  pipe  with  the  pump.  The 
pump  is  started  ;  the  water  rushes  through  the  small  apertures 
at  the  bottom,  and  passes  upward  between  the  two  pipes, 
bringing  the  material  of  the  bottom  with  it,  which  is  discharged 
at  the  top.  The  small  pipe  is  turned,  and  easily  works  its  way 
into  the  soil.  The  larger  pipe  would  also  settle  with  it ;  but 
this  is  not  necessary,  and  is  fastened  at  the  top.  At  intervals 
of  4  or  5  ft.  the  inner  pipe  can  be  lifted  entirely  out,  the  chisel 
end  removed  ;  a  tube  of  iron  or  brass  a  foot  long,  slightly  con¬ 
tracted  at  the  lower  end,  of  the  size  and  shape  of  the  straight 
cylindrical  lamp-chimney,  is  now  screwed  to  the  small  pipe, 
lowered  to  the  bottom  of  the  hole,  and  pressed  1  foot  into  the 
bottom,  then  lifted  out  again.  The  material  in  the  brass  tube 
can  now  be  pressed  by  means  of  a  round  stick  into  a  lamp- 


SOUNDINGS  OR  BORINGS. 


169 


chimney,  a  piece  of  thin  sheet  rubber  fastened  over  the  ends. 
We  have  now  a  cylindrical  specimen  of  the  material  at  that 
depth  in  the  exast  condition  in  which  it  existed  in  the  strata  ; 
this  will  retain  its  moisture  for  a  long  time.  This  is,  however, 
only  practicable  in  clay,  silt,  or  mixed  soils.  In  these  materials 
the  hole  will  remain,  and  the  pipe  can  be  lowered  to  its  bottom 
without  letting  the  larger  pipe  follow.  We  have  often  bored 
40  to  50  ft.  in  a  day.  When  in  a  bed  of  loose  sand  and  gravel, 
the  water  will  not  return  upwards  through  the  annular  space 
between  the  pipes,  but  will  escape  laterally  at  the  bottom,  so 
that  the  material  cannot  be  brought  to  the  surface,  but  the 
water  will  so  loosen  it  that  the  pipe  will  readily  sink.  It  should, 
however,  be  turned  constantly  and  rapidly,  and  be  lifted  a  short 
distance  occasionally  ;  otherwise  the  sand  and  gravel  will  rise 
up  between  the  two  pipes,  or  compact  above  the  chisel  section 
and  bind  the  pipe,  increasing  both  the  difficulty  of  sinking  or 
raising  the  pipe,  and  causing  sometimes  the  loss  of  the  smaller 
pipe.  It  is  better  in  this  material  to  let  or  make  the  larger 
pipe  follow  the  smaller  one,  but  it  is  not  necessary.  The  only 
skill  required  in  this  method  of  boring  is  to  prevent  this  bind¬ 
ing.  In  silt  there  is  danger  of  the  pipe  sinking  too  rapidly,  which 
would  also  bind  the  pipe  ;  constant  turning  and  frequent  lifting 
will  prevent  it  in  either  case.  If  bowlders  are  encountered,  or 
logs  either,  they  can  be  readily  drilled  through,  as  the  pipe  is 
free  to  be  lifted  and  dropped.  Rock  is  easily  determined  either 
by  hammering  on  the  top  of  the  pipe  or  lifting  it  and  letting 
it  drop;  the  rebound  or  the  sound  either  enables  you  to  dis¬ 
tinguish  between  solid  rock  and  bowlders.  The  entire  outfit  is 
cheap;  any  plumber  or  mechanic  can  make  the  connections 
necessary  with  the  ordinary  gas  or  water  pipes  in  common  use. 
Two  or  three  men  can  perform  all  of  the  wmrk  required.  This 
method  was  used  in  all  of  the  piers  at  the  Susquehanna  River, 
Schuylkill,  and  Tombigbee,  and  Ohio  River  at  Louisville,  in 
depths  varying  from  40  to  over  100  ft.  A  single  boring  at  the 
site  of  a  pier  is  not  enough  ;  at  the  Susquehanna  and  Schuylkill, 
from  six  to  ten  soundings  were  made  at  the  site  of  each  caisson, 
one  at  each  corner,  and  one  or  two  intermediate  along  the 


170  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


sides,  and  if  any  great  or  abrupt  irregularities  were  developed 
additional  borings  made.  From  this  an  exact  diagram  of  the 
lay  of  the  bottom,  in  reference  to  the  surface  of  the  water  and 
bed  of  the  river,  was  made.  VVe  knew  in  the  beginning  the 
exact  nature  of  the  material  to  be  passed  through,  the  high 
and  low  points  of  the  rock  and  the  exact  depth  to  each,  and 
the  exact  amount  of  material  to  be  excavated.  The  position 
of  each  sounding  was  located  accurately  by  triangulation  or  the 
wire  measurement  from  an  established  base.  At  the  Susque¬ 
hanna  a  good  part  of  the  boring  was  done  in  the  winter,  when 
the  river  was  solidly  frozen  over.  The  information  thus  ob¬ 
tained  cost  a  very  small  sum,  and  was  invaluable.  (See  Fig, 
io,  Plate  IV.) 


Article  XXXIV. 

TIMBER  PIERS. 

28.  WHERE  stone  or  brick  is  hard  to  obtain,  or  costly  on 
account  of  the  long  distances  over  which  it  must  be  hauled  or 
transported,  or  when  it  is  important  to  erect  a  bridge  without 
the  delay  incident  to  constructing  masonry  piers,  it  becomes 
necessary  to  build  timber  piers.  These  piers  are  generally  so 
designed  that  masonry  or  iron  piers  can  be  easily  and  con¬ 
veniently  constructed  at  some  future  time.  Two  types  of  such 
piers,  constructed  by  the  writer,  will  be  briefly  described  and 
illustrated.  In  building  a  railroad  across  the  swamps  in 
Alabama  between  Tensas  and  Mobile,  a  number  of  bayous  or 
streams  had  to  be  crossed  :  these  being  navigable,  many  draw¬ 
bridges  had  to  be  constructed,  large  enough  to  pass  the  large 
steamboats  navigating  the  main  rivers.  The  pivot  or  centre 
piers  were  constructed  by  driving  a  number  of  piles  ft.  cen¬ 
tres  over  a  square  area  of  the  proper  size  ;  these  piles  were  cut 
off  a  few  feet  above  the  water  surface,  were  capped  with 
12  X  12  in.  square  timber  and  bolted  to  the  piles  with  drift 
bolts  1  in.  square  and  2  ft.  long.  Upon  these,  and  at  right 
angles  to  them,  other  timbers  were  placed  at  about  one  foot 


TIMBER  PIERS. 


171 


intervals,  and  upon  these  a  solid  flooring  of  square  timbers  ; 
these  timbers  were  connected  by  i-in.  screw  bolts  with  2-ft.  grip 
— that  is,  length  of  bolt  between  timber  surfaces  ;  the  entire 
length  from  out  to  out  of  bolt,  included  head,  nut,  and  washers, 
would  be  2  ft.  3^  in.  This  completed  the  pier;  the  upper 
surface  was  then  planed  to  a  level,  to  receive  the  turn-table 
arrangements.  The  number  of  piles  required  varied  from  49 
to  81,  according  to  the  size  of  the  piers,  and  were  driven  from 
30  to  40  ft.  into  the  bed  of  the  stream,  which  was  composed 
almost  entirely  of  a  soft  silt  or  mud.  The  rest-piers  were  com¬ 
posed  of  three  rows  of  piles,  about  3  ft.  apart,  from  5  to  6  piles 
in  each  row  ;  these  were  capped  with  three  courses  of  timber, 
as  in  the  pivot  pier ;  upon  this  flooring  other  timbers  were 
placed  to  support  the  latch  beam  for  the  ends  of  the  draw  span, 
and  for  the  support  of  the  stringers  of  the  trestle  approach. 
These  piers  were  now  complete  ;  they  were  constructed  in  a 
few  days  and  at  a  small  cost.  It  was  an  easy  matter  at  some 
subsequent  time  to  cut  the  piles  off  below  low  water,  and  to 
build  brick  piers  on  them  ;  this  has  been  done. 

29.  The  following  is  another  form  of  timber  pier  carrying 
spans  from  100  to  120  ft.  Sixteen  piles  were  driven  in  two  rows, 
eight  piles  in  each  row.  The  distance  between  the  rows  was 
regulated,  as  in  masonry  piers,  by  the  size  required  at  the  top, 
and  allowing  for  the  batter.  The  distance  between  the  piles 
in  each  row  was  regulated  by  placing  the  piles  to  the  best  ad¬ 
vantage  for  supporting  the  structure  above,  as  will  be  seen  in 
the  drawing;  these  were  then  capped  with  12  X  12  in.  timber, 
upon  which  were  placed  other  timbers  at  right  angles  to  the  first, 
all  bolted  and  fastened  together  by  iron  straps.  Upon  this  plat¬ 
form  a  strong  double  framed-trestle  was  erected,  constructed 
of  eight  vertical  and  inclined  posts,  with  cap  and  sill ;  on  these 
cross-pieces  were  placed,  and  then  a  platform  for  the  bridge  to 
rest  upon.  All  timbers  were  12  X  12  in.  square,  the  whole 
structure  well  tied  together,  and  braced  longitudinally.  See 
drawing,  Figs.  1,  2,  and  la,  Plate  VI.  Such  a  structure  will 
bear  a  heavy  load,  is  however,  temporary  in  its  nature,  is 
very  light,  and  is  wanting  in  stability  if  liable  to  be  subjected 


172 


A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


to  severe  shocks.  In  this  case  a  timber  starling  or  cut-water 
should  be  constructed  at  the  up-stream  end,  either  built  en¬ 
tirely  separate  from  the  pier  itself  or  forming  a  part  of  it. 
Extra  piles  can  be  driven  triangular  in  plan,  and  resting  upon 
these  large  square  timbers  should  be  placed,  inclining  at  a 
rather  flat  angle  to  the  body  of  the  pier;  this  should  be 
close-sheeted  with  plank,  and  filled  in  part  or  entirely  with 
broken  stone  or  gravel,  or  better  with  concrete — in  which  event, 
when  the  timbers  rot,  a  permanent  concrete  pier  would  remain. 
Either  pine  or  oak  timber  is  well  suited  for  such  structures. 


Article  XXXV. 

FRAMED  TRESTLES. 

30.  TRESTLES  are  used  to  carry  a  railroad  over  swamps, 
rivers,  or  creeks,  when  material  for  making  embankments  is 
difficult  to  obtain.  In  building  over  low  places,  a  simple 
framed  trestle  composed  of  a  cap,  sill,  and  four  posts,  two  placed 
vertically  under  the  rails  and  one  on  either  side  inclined  to  the 
verticals,  with  a  batter  of  about  3  in.  to  each  vertical  foot. 
This  frame  ordinarily  rests  on  mud  sills,  which  are  simply 
short  pieces  of  square  timber  5  or  6  ft.  long,  partly  imbedded 
in  the  soil,  or  better  on  small  rubble-masonry  pedestals.  All 
timbers  in  these  frames  or  bents  are  generally  12  X  12  in.  tim¬ 
ber ;  the  batter  posts  are  sometimes  10  X12  in.  square.  Diag¬ 
onal  bracings,  called  X-bracing,  are  then  placed  passing  diago¬ 
nally  from  the  top  to  the  bottom  of  the  bent  on  both  sides, 
made  of  3-in.  X  9  or  12-in.  plank,  and  bolted  to  each  post. 
Spikes  are  often  used  ;  bolts  cost  but  little  more,  and  are  much 
better  and  ultimately  more  economical.  If  the  bents  are  very 
high,  longitudinal  braces  either  of  3-in.  plank  or  6  X  6  in.  square 
timber  are  placed  from  bent  to  bent  and  bolted  to  the  posts. 
The  bents  are  placed  generally  from  12^  ft.  to  14  ft.  centre  to 
centre.  Where  a  greater  height  than  25  ft.  is  required  for  the 
bents,  they  are  built  with  one,  two,  or  more  stories,  each  of  a 


FRAMED  TRESTLES. 


173 


height  not  exceeding  20  ft.,  the  upper  section  resting  on  top  of 
the  next  lower,  and  so  on  to  the  bottom,  the  posts  in  each  section 
framed  so  as  to  be  in  the  prolongation  of  those  in  the  sections 
above,  and  additional  verticals  or  inclined  pieces  are  introduced 
in  the  lower  sections,  the  whole  thoroughly  bolted  and  braced 
together.  In  the  form  of  trestle  just  described,  the  vertical 
posts  are  supposed  to  carry  the  greater  part  of  the  load,  the 
batter  or  inclined  posts  mainly  intended  to  give  a  wide  base 
and  lateral  stiffness  to  the  bent.  This  is  the  actual  case,  when 
the  top  of  the  batter  post  is  a  foot  or  more  from  the  top  of  the 
vertical.  When  placed  close  together  both  posts  bear  a  part  of 
the  load.  In  order  to  give  stiffness  and  at  the  same  time  to 
make  all  posts  bear  an  equal  share  of  the  load,  all  of  the  posts 
are  inclined  at  or  about  the  same  angle ;  this  is  called  the  M- 
trestle  ;  two  posts  touching  at  top  and  inclining  downwards  in 
opposite  directions  are  placed  under  each  rail,  the  inner  posts 
coming  together  at  the  bottom  in  the  middle  ;  the  outside  posts 
may  have  a  little  greater  batter  than  the  inner  ones.  In  this 
manner  the  load  is  distributed  over  and  borne  by  four  instead 
of  two  posts.  In  this  form  of  trestle  the  posts  need  not  be  more 
than  9  or  10  X  12  ins.  square  ;  but  unless  the  amount  of  timber 
required  is  very  great,  as  in  very  high  and  long  trestles,  the 
timbers  are  not  proportioned  to  the  loads  they  have  to  bear,  as 
the  amount  of  material  saved  would  be  of  little  moment.  In 
very  high  trestles,  such  as  two,  three,  or  more  stories,  economy 
requires  the  bents  to  be  placed  farther  apart,  the  timbers  in 
the  bents  remaining  from  10  to  12  X  12  ins.  square.  In 
high  trestles  the  bracing  and  some  of  the  auxiliary  or  second¬ 
ary  members  are  of  smaller  scantling  or  plank.  The  drawings, 
showing  in  detail  the  types  of  the  different  trestles  and  the 
dimensions  of  the  several  members  used,  there  will  be  no  need 
of  a  lengthy  description.  (See  Figs.  3  and  4,  Plate  VI.) 

31.  It  will  be  sufficient  to  say  that  the  upper  cap  need  not 
be  over  10  or  12  feet  long.  The  intermediate  sills  (which  are 
a  sill  to  the  section  above  and  a  cap  to  the  section  below)  are 
equal  to  the  length  of  the  cap  increased  by  6  inches  for  each 
vertical  foot  between  them,  to  these  the  posts  are  connected 


174  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 

by  the  mortise-and-tenon  joint,  or  by  drift-bolts  or  iron  straps, 
— commonly  by  the  first  method.  Some  engineers  use  two 
ntermediate  pieces,  so  that  each  section  is  a  separate  and  in¬ 
dependent  frame,  in  which  case  the  longitudinal  braces  are 
framed  between  the  two.  There  seems  to  be  no  decided  ad¬ 
vantage  in  this,  and  it  requires  more  material,  and  consequently 
more  cost.  The  bottom  sill  will  be  equal  to  the  top  cap  in¬ 
creased  by  6  inches  multiplied  by  the  total  height  of  the  tres¬ 
tle.  All  the  main  posts  should  be  in  lengths  of  about  20  feet, 
except  the  bottom  section,  which  is  generally  less  than  20  feet, 
often  not  over  from  5  to  10  feet.  The  longitudinal  braces  are 
from  6  X  6  inches  to  6  X  8  inches  square,  and  the  same  for 
the  secondary  members.  The  diagonal  bracing  is  generally 
3-inch  plank,  9  to  12  inches  wide.  (See  Plate  VII,  Figs.  1,  2,  3, 
and  4.) 

32.  Whatever  may  be  the  height  and  form  of  trestle-bent, 
and  whatever  distance  apart,  there  are  placed  on  top  of  the  cap, 
stringers,  extending  longitudinally  and  at  right  angles  to  the 
cap,  one  stringer  under  each  rail,  and  commonly  placed  di¬ 
rectly  over  a  post.  Each  stringer  is  made  of  two  pieces  for 
bents  12^  feet  centres.  They  are  6  X  14  inches  cross-section 
each,  and  should  be  25  feet  long.  They  are  placed  side  by 
side  with  a  2  or  3  inch  space  between  them,  and  bolted  to¬ 
gether  by  screw-bolts,  blocks  of  wood  or  cast-iron  spools  being 
placed  between  them,  through  which  the  bolts  pass.  These 
bolts  are  from  f  to  £  inch  diameter,  with  cast-iron  or  wrought 
plate  washers,  and  nuts  to  suit.  Four  bolts  are  used  over  each 
cap,  and  sometimes  two  or  more  between  the  bents,  for  each 
packed  stringer,  and  one  bolt  from  £  to  1  inch  diameter  to 
bolt  the  stringer  to  the  cap.  This  bolt  has  to  be  slightly 
inclined  so  as  to  pass  the  end  of  the  post  below.  For  bents 

14  to  15  feet  apart,  packed  stringers  7X15  inches  or  8  X  15 
inches  are  used,  and  if  practicable  to  obtain  them  they  should 
be  28  or  30  feet  long;  but  if  not  practicable,  lengths  of  14  or 

15  feet  can  be  used.  But  this  requires  a  bolster,  that  is,  a 
piece  of  timber  8  to  10  inches  thick,  4  to  5  feet  long,  placed 
on  the  cap  and  bolted  to  it.  The  stringers  then  rest  on  the 


FRAMED  TRESTLES. 


175 


bolsters,  to  which  they  are  bolted.  In  this  case  the  stringers  do 
not  break  joint  on  the  caps.  It  does  not  form  as  stiff  a  trestle 
as  when  the  joints  are  broken,  but  will  answer  every  purpose. 
Pieces  of  timber  bolted  to  the  side  of  the  stringers  and  rest¬ 
ing  on  the  cap  can  be  used  instead  of  the  bolsters.  (See  Plate 
VII.) 

33.  For  spans  from  15  to  25  feet  it  is  economical  to  use 
stringers  of  the  same  cross-section  as  above,  but  they  should  be 
strengthened  below  either  by  struts  resting  on  the  bents  at  the 
lower  end  and  at  the  upper  end  against  straining-pieces — these 
are  pieces  of  timber  bolted  to  the  stringers  underneath  and  near 
the  centre,  these  pieces  being  5  or  6  feet  long,  and  6  or  8  inches 
thick — or  by  iron  rods  passing  through  the  stringers  at  the  ends, 
and  under  a  block  of  wood  or  a  short  iron  strut  placed  under 
and  at  the  centre  of  the  length  of  the  stringer.  Either  of  these 
constitute  a  trussed  beam.  The  stringers  in  this  case  can  be 
either  of  two  or  three  pieces  packed  together.  Sometimes,  in¬ 
stead  of  trussing  the  stringers  as  above,  an  increased  number  of 
pieces  can  be  used.  Four  pieces  of  timber  12  X  12  inches 
square,  bolted  together  with  bolts  and  packing-blocks,  which 
are  framed  into  the  sticks  so  as  to  act  as  keys,  will  be  amply 
strong  for  a  span  from  20  to  25  feet ;  or  two  pieces  laid  side  by 
side,  and  another  single  piece  on  top,  will  do  for  a  span  from 
15  to  20  feet.  These  are  mere  expedients,  have  a  bad  distri¬ 
bution  of  material,  and  make  a  heavy,  bungling-looking  job. 
See  Figs.  1  a  to  6a,  Plate  VIII.) 

34.  On  top  of  the  stringers  the  cross-ties  are  laid.  These 
are  6x8  inches  square,  and  from  8^  to  9^-  feet  long,  are  gen¬ 
erally  spaced  from  12  to  16  inches  centres,  and  spiked  to  the 
stringers.  It  is  poor  economy  to  space  the  ties  with  too  great 
intervals.  If  a  train  runs  off  the  track  the  wheels  sink  in  the 
space  between  the  ties,  and  before  the  momentum  can  be 
overcome,  the  trestle  will  be  torn  to  pieces,  and  not  unfre- 
quently  the  train  will  be  badly  wrecked.  If  the  ties  are  not 
over  2  or  3  inches  apart,  the  train  can  easily  run  on  the  ties 
without  serious  damage  to  either  train  or  trestle.  On  top  of 
the  ties  and  near  their  outer  edges  guard-rails  (longitud- 


1 76  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


inal  pieces  of  timber  6x6  inches  or  6  X  8  inches)  are  placed 
and  bolted  to  the  ties.  A  common  rule  is  to  use  a  ^-inch 
screw-bolt  at  every  fifth  tie,  and  spikes  between  the  bolts. 
The  guard-rails  are  sometimes  cut  or  “  dapped  ”  to  the  depth  of 
an  inch,  a  tongue  projecting  down  between  the  ties.  What¬ 
ever  theoretical  advantage  may  exist  practically  as  the  work  is 
done,  there  is  no  advantage  whatever  gained.  The  writer 
thinks  that  the  guard-rail  had  better  be  placed  inside  and 
close  to  the  rail,  not  over  two  or  three  inches  from  the  rail. 
In  this  case  the  top  of  the  guard-rail  should  not  be  above  the 
iron  rail  of  the  track.  It  might  be  placed  both  inside  and  out¬ 
side.  This  completes  the  trestle.  The  cross-ties  on  trestles 
and  bridges  are  always  sawed.  On  embankments  they  are 
hewn.  It  is  claimed  that  a  hewn  tie  will  last  longer  than  a 
sawed  tie.  No  good  reason  seems  to  be  assigned  for  this. 
The  hewn  tie  is  almost  universally  used  on  embankments,  as 
they  are  generally  cut  and  hauled  from  the  woods  adjacent  to 
the  line,  carried  by  trains  to  point  where  used  ;  smaller  trees 
are  used,  which  are  younger  and  more  vigorous  than  those 
which  are  larger  and  on  the  decline,  and  which  have  to  be 
split  through  the  centre.  I11  the  piney  woods  of  the  South 
dead-wood  trees,  either  standing  or  fallen,  are  largely  used  for 
ties,  and  are  said  to  be  equally  as  good  as,  if  not  better  than, 
green-wood  ties,  as  the  sap  is  either  rotted  or  is  easily  removed. 
Oak  ties,  either  white  oak  or  post  oak,  are  preferred  to  the 
pine  ties.  They  are  not  so  rapidly  cut  into  by  the  rail,  this 
cutting  tending  to  cause  rot  in  the  tie  under  the  rail,  and  they 
hold  the  spike  better,  owing  to  their  hardness  ;  but,  on  the 
contrary,  they  warp  and  split  to  a  greater  extent  than  pine, 
which  admitting  and  holding  water  causes  internal  rot.  Oak 
ties  cost  about  10  cents  more  than  pine  ties,  40  cents  and  30 
cents,  respectively,  being  average  costs.  Pine  is  generally  used 
where  it  is  abundant ;  oak,  where  it  is  abundant.  The  same 
may  be  said  in  general  of  the  other  timbers  of  the  trestle. 
But  probably  pine  is  preferred  for  caps,  sills,  and  posts.  It  is 
more  easily  cut  and  framed,  has  great  strength  and  durability, 
does  not  warp,  split,  or  crack  as  readily  as  oak,  and  can  be 


FRAMED  TRESTLES. 


177 


obtained  in  longer  and  straighter  sticks.  Oak  is  somewhat 
stronger  and  heavier.  Either  will  answer  in  any  framed  struc¬ 
ture.  Cost  and  rapidity  of  supply  alone  are  factors  worth 
considering  in  deciding  between  the  two. 


Article  XXXVI. 

QUALITIES  OF  TIMBER. 

35.  In  the  North  white  pine  is  abundant,  and  is  used  'for 
all  purposes  in  framing.  It  is  soft  and  white,  easily  worked, 
and  possesses  great  strength  and  durability.  In  the  South  the 
long-leaf  yellow  pine  is  found  in  abundance  ;  being  harder  and 
generally  claimed  to  be  stronger  than  the  white  pine,  it  is  used 
for  all  purposes  of  construction,  and  can  be  obtained  in  long, 
straight  sticks  or  logs,  free  from  knots  and  many  other  defects. 
There  exist  two  apparently  distinct  species,  presenting  practi¬ 
cally  the  same  outside  appearances.  The  one,  however,  has  a 
large  proportion  of  sap-wood  and  a  small  proportion  of  heart 
wood.  This  should  not  be  used  in  structures  unless  immersed 
under  water  at  all  times.  The  other  has  very  little  sap,  and 
long,  large  straight  pieces  can  be  obtained  of  almost  clear  heart. 
Such  timber  is  unsurpassed  for  use  in  structures  above  ground, 
exposed  to  alternate  wetness  and  dryness.  In  middle  latitudes 
a  pine  grows,  called  commonly  spruce  pine  ;  is  used  freely 
where  it  grows,  but  is  not  considered  equal  to  either  of  the 
above  species  in  strength  or  durability.  Nor  is  it  in  such 
abundance,  but  grows  scattered  over  the  ordinary  forests  in 
greater  or  less  quantities,  often  high  up  on  the  sides  of  moun¬ 
tains,  difficult  to  cut  and  haul,  and  is  soon  exhausted  in  any 
particular  locality. 

36.  For  many  years  the  pine  trees  of  the  South  have 
been  drained  of  their  resinous  matter  yearly,  called  bleeding 
or  turpentining.  This  is  done  by  boxing  the  tree  ;  that  is,  cut¬ 
ting  a  bucket-shaped  notch  near  the  bottom,  and  a  series  of 
channels  or  grooves  leading  into  it.  The  resinous  matter  drips 
into  this  bucket,  and  is  removed  at  regular  intervals.  How 


178  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 

far  and  to  what  extent  this  affects  the  growth,  ultimate  hard¬ 
ness,  strength,  and  durability  of  the  timber,  is  not  perhaps 
known.  It  would  certainly  seem  to  affect  the  tree  injuriously 
in  all  of  the  above  respects,  and  it  is  not  uncommon  to  specify 
that  no  “  bled  ”  timber  shall  be  used  in  trestles,  bridge  timbers, 
or  other  structure  above  ground.  The  owners  of  the  forests 
stoutly  maintain  that  it  does  not  hurt  or  injure  the  timber, 
and  the  saw-mill  owners  generally  side  with  them.  Both  are 
interested  parties,  as  the  one  gets  a  double  profit  from  the 
standing  trees,  and  the  second  from  the  sawing.  But  whether 
it  injures  the  timber  or  not  is  of  but  little  practical  value,  as 
the  “  bleeding  ”  is  almost  universal,  and  it  is  almost  impos¬ 
sible  to  make  large  contracts  under  this  restriction;  if  you  do, 
there  is  no  means  of  detecting  the  difference  between  the 
two ;  as  a  consequence,  you  may  have  to  pay  high  for  the 
requirement,  and,  after  all,  only  get  the  bled  timber.  Some 
carpenters  or  experts  say  that  they  can  distinguish  the  differ¬ 
ence.  Some  recent  experiments  have  been  made,  and  it  has 
been  reported  that  the  bled  timber  is  equal,  if  not  superior,  to 
the  unblecl  ;  but  this  can  only  be  fully  settled  after  years  of 
experiment. 

37.  Oak  timber  flourishes,  apparently,  in  any  country  or 
section  of  the  country,  and  possesses  great  hardness  and 
strength,  as  well  as  durability,  but  does  not  attain  the  same 
height  ;  is  not  as  straight,  is  rarely  free  from  knots  of  large 
sizes,  and  cannot  be  obtained  in  as  long,  straight  pieces  as  the 
pines,  but  is  largely  used  in  many  parts  of  the  country.  Its 
properties  depend  largely  upon  the  nature  of  the  soil  in  which 
it  grows,  that  which  grows  in  low  or  swampy  lands  being  far 
inferior  to  that  grown  in  the  same  latitude  and  same  climate, 
but  on  the  higher  grounds.  The  first  is  soft  and  soppy,  and  has 
not  the  strength  or  durability  of  the  second,  and  should  not 
be  used  above  the  ground.  Post  oak,  so  called,  seems  to  be  a 
small  white  oak,  but  is  considered  even  harder  and  stronger 
than  the  white  oak  ;  but  does  not  attain  as  great  size,  and  is 
used  principally  for  ties  and  other  structures  requiring  small 
cross-sections  and  lengths.  Of  the  several  kinds  of  oak,  white, 


FRAMED  TRESTLES. 


1 79 


red,  black,  post,  and  live  oak,  the  white  and  post  oaks  are 
alone  used  in  ordinary  structures.  The  live-oak  is  the  hardest, 
strongest,  and  most  durable  timber  in  the  United  States;  is 
owned  and  reserved  by  the  Government,  and  is  only  used  in 
ship-building  and  other  governmental  structures. 

38.  Cypress  timber  grows  to  a  great  extent  in  the  Southern 
States  ;  is  durable,  and  exceeds  even  the  pines  in  this  respect, 
as  well  as  in  the  lengths  and  diameters  of  the  trees.  It  has  not 
the  strength  of  the  other  timbers  mentioned,  but  is  often  used 
for  piles  and  for  the  various  parts  of  the  trestles,  but  in  some¬ 
what  larger  dimensions  when  required  to  carry  the  same  loads. 
Owing  to  the  ease  with  which  it  splits  into  thin  slabs,  and  its 
remarkable  durability  when  exposed,  it  is  used  to  a  large  ex¬ 
tent  in  making  shingles,  staves,  weather-boarding,  and  the  like. 
There  is  a  species  of  this  timber  called  black  cypress,  which 
has  a  durability  equal  to  that  of  any  timber.  It  is,  however, 
rather  scarce,  and  is  rather  small  in  diameter,  but  is  much 
sought  after  for  the  piles  of  trestles  in  the  Southern  swamps. 
It  is  certainly  far  superior  to  the  pines  in  durability. 

39.  The  writer  does  not  mention  the  chestnut,  poplar,  elm, 
cedar,  etc.,  as  these  are  seldom  found  in  such  structures  as  are 
embraced  in  this  work,  though  possessing  many  valuable  and 
useful  properties,  and  are  largely  used  for  many  purposes  re¬ 
quiring  durability,  ease  of  working,  and  where  no  great  strength 
is  required. 

40.  There  are  a  few  general  principles  which  will  be  of  ser¬ 
vice  in  determining  the  general  properties  of  the  various  species 
of  timber  trees. 

1.  The  heaviness  generally  indicates  good  timber,  as  does 
also  the  darkness  in  color.  2.  The  slowness  of  growth,  as  in¬ 
dicated  by  the  narrowness  and  closeness  of  the  annual  rings. 
3.  The  larger  the  proportion  of  heart  timber,  which  is  gener¬ 
ally  distinguished  from  the  sap-wood  by  a  darker  color,  and  a 
harder,  firmer  material,  the  better  the  timber.  4.  All  timber 
grown  on  a  sandy,  elevated,  well-drained  area  is  superior  to 
that  grown  on  very  low  and  swampy  ground.  5.  Of  the  same 
species,  that  grown  in  a  cold  climate  is  generally  considered  the 


l80  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 

best,  whereas  of  different  species  the  best  is  grown  in  a  warnre 
climate.  But,  fortunately,  such  timbers  as  grow  in  this 
country,  whether  North  or  South,  and  on  almost  any  kind  of 
soil,  except  a  very  swampy  soil,  that  can  be  obtained  in  the 
requisite  lengths  and  sizes  for  large,  heavy  structures,  have 
the  necessary  strength,  and  are  not  materially  different  in 
point  of  durability.  Our  choice  is  largely  controlled  by 
circumstances. 


Article  XXXVII. 

DURABILITY  OF  TIMBER. 

41.  The  average  life  of  timber  when  exposed  will  rarely  ex¬ 
ceed  from  eight  to  ten  years,  and  unless  covered  or  preserved 
by  artificial  means  of  some  kind,  all  structures  should  be  entirely 
renewed  in  that  time.  The  same  timbers  under  apparently 
the  same  conditions  are  very  variable  in  point  of  durability, — 
this  may  arise  from  one  or  more  causes,  which  will  be  alluded 
to  in  another  paragraph, — and  a  structure  will  begin  to  show  de¬ 
cided  evidence  of  decay  in  some  of  its  parts  in  from  two  to  four 
years.  This  can  only  be  discovered  by  careful  and  continuous 
inspection.  The  renewal  should  therefore  commence  at  an 
early  day  and  be  continued  as  the  case  demands  until  entirely 
renewed,  and,  in  fact,  renewal  and  repairs  should  be  practically 
continuous.  It  is  due  to  the  neglect  to  do  this  that  many  of 
our  most  serious  accidents  can  be  traced.  The  bulk  of  the 
timber  used  on  railroads  is  consumed  in  building  trestles,  for 
both  rapidity  of  construction  and  decrease  of  first  cost  often 
prove  the  impracticability  of  obtaining,  on  the  first  construc¬ 
tion  of  a  road,  a  more  durable  material.  Therefore  on  almost 
all  new  roads  many  very  long  and  very  high  trestles  are  con¬ 
structed.  These  are  called  and  regarded  as  temporary  struct¬ 
ures,  and  are  frequently  not  built  with  as  much  care  and  strength 
as  they  would  be  if  intended  to  remain  permanently  as  timber 
structures.  Financial  considerations  prevent  the  substitution 
of  earthen  embankments,  masonry  arches  or  abutments,  or 


FRAMED  TRESTLES. 


181 


iron  viaducts, — this  is  intended  to  be  done  from  year  to  year, — 
and  the  old  trestles  must  be  maintained  at  a  minimum  cost  for 
repairs.  Rotten  stringers  and  posts  are  patched  up  by  spiking 
or  bolting  strips  or  planks  to  them,  not  unfrequently  doing  more 
harm  than  good.  Heavier  engines  and  trains  are  run  over  them, 
taking  the  risk,  until  finally  the  structure  collapses,  resulting  in 
great  loss  of  life  or  property  ;  and  so-called  experts  are  called  in 
to  explain  the  causes  of  the  disaster.  These  often  satisfy  courts 
and  juries,  but,  in  fact,  the  structure  has  simply  collapsed  from 
inherent  weakness  on  account  of  rot. 

42.  Weakness  resulting  from  rot  generally  arises  at  the 
joints,  where  timber  rests  on  timber — as  the  surfaces  of 
contact  between  tie  and  stringer,  between  stringer  and  cap, 
cap  and  post,  post  and  sill.  The  deterioration  does  not  take 
place  first  where  it  can  be  seen,  but  well  in  and  under  the  top 
piece.  The  entire  stringer  may  show  a  hard,  firm,  sound  sur¬ 
face,  yet  in  unseen  parts  it  may  be  rotten  to  the  core. 
(Simple  knocking  on  the  surface  does  not  always  indicate  the 
exact  condition  of  the  interior.  The  writer  has  seen  this  tried 
many  times,  and  erroneous  reports  made  thereon.  The  only 
satisfactory  test  is  to  bore  into  the  timber  with  a  gimlet  or 
auger,  the  bit  not  exceeding  ^  inch  diameter.)  It  is  easy  to 
understand  why  this  is  the  case.  The  moisture  on  an  exposed 
surface  evaporates  rapidly  under  the  influence  of  the  sun  and 
wind,  whereas  that  portion  which  lodges  in  the  joints  between 
the  timbers  remains  for  a  longer  time,  and  if  it  has  access  to 
the  ends  of  the  timber  it  will  spread  for  a  considerable  distance 
in  the  direction  of  the  length  of  the  stick.  The  moisture  and 
internal  heat  combined  with  all  the  conditions  favorable  to 
decay,  rot  will  take  place  in  the  interior  of  the  stick  and 
under  the  top  stick.  This  is  the  condition  with  the  ends  of 
the  stringer,  the  top  and  bottom  of  the  posts,  and  to  a  less 
degree  under  the  ties.  Stringers  may  be  perfectly  sound  in 
the  middle  of  their  lengths,  and  rotted  to  a  dangerous  degree 
at  the  ends,  especially  on  the  under  side.  The  same  in  the 
posts.  The  writer  has  examined  miles  upon  miles  of  old 
trestles,  boring  into  the  timbers  at  the  ends  and  at  one  or  two 


A  PRACTICAL  TREATISE  ON  FOUNDATIONS 

intermediate  points,  and  rarely  has  he  observed  any  variation 
from  the  above  rule.  The  mortise-and-tenon  joint  is  partic¬ 
ularly  favorable  to  bring  about  the  above  injurious  conditions. 
This  will  be  further  alluded  to  in  discussing  the  subject  of 
joints.  On  examining  an  old  structure,  bore  into  the  ends  of 
the  stringers,  the  top  and  bottom  of  the  posts,  and  obliquely 
under  the  ties  at  the  middle  of  the  stringer.  If  no  signs  of 
decay  are  thus  discovered  you  may  be  satisfied  that  the  rest  of 
the  piece  is  sound,  unless  unseen  defects  existed  in  the  begin¬ 
ning,  which  generally  should  be  seen  on  the  outside;  and  this 
timber  should  have  been  originally  condemned.  Oak  timber 
sometimes,  when  standing,  is  rotten  in  the  centre,  but  this  will 
generally  be  discovered  in  cutting  down  the  tree,  or  in  the 
subsequent  sawing  of  the  loo-. 

0  b 


Article  XXXVIII. 

MEANS  OF  PRESERVING  TIMBER. 

43.  Timber  should  be  free  from  such  defects  as  cracks, 
radiating  from  the  centre,  or  cracks  which  separate  one  annual 
layer  from  another,  splitting  as  it  were  into  rings,  or  upsets 
where  the  fibres  have  been  injured  by  compression,  or  large 
knots.  Doty  or  spongy  places  indicate  incipient  decay. 
Almost  all  timber  will  show  cracks  when  exposed,  after  bein<r 
cut,  in  the  green  state,  to  the  sun  or  high  winds.  If  these  only 
extend  to  a  short  distance  into  the  timber,  and  for  short  dis¬ 
tances  in  the  direction  of  its  length,  no  especial  notice  need  be 
taken  of  them,  for  the  cracks  are,  in  fact,  unavoidable ;  but  if 
they  extend  half  the  way  or  all  of  the  way  through  a  stick,  it 
should  be  condemned,  as  they  materially  weaken  the  timber, 
and  in  addition  will  aid  in  hastening  rot.  Large  knots  also 
weaken  the  timber,  as  they  do  not  adhere  strongly  to  the  sur¬ 
rounding  fibres,  and  in  addition  the  fibres  themselves  around 
the  knots  are  upset  or  crippled.  The  knots  also  in  time  fre¬ 
quently  become  loose  and  separate  from  the  surrounding 
timber.  A  limited  number  of  small  knots,  if  not  occurring  in 


MEANS  OF  PRESERVING  TIMBER. 


183 


the  same  vertical  or  horizontal  plane,  or  not  extending  entirely 
through  the  stick,  need  not  be  a  serious  objection.  A  little 
sap  on  the  corners  of  large  sticks  has  practically  the  effect  of 
reducing  the  area  in  proportion  to  the  amount  of  sap,  as  the 
sap  rapidly  rots.  It  is  almost  impossible  to  secure  large-sized 
timbers  in  great  quantities  entirely  free  from  all  of  the  above 
defects,  but  the  right  to  limit  the  defects  should  be  reserved 
by  the  engineer.  This  is  done  by  specifying  that  the  timber 
shall  be  clear-heart,  and  free  from  any  of  the  above  defects. 
The  material  must  be  either  condemned  at  the  mills  or  on  de¬ 
livery  at  the  site  of  the  work. 

44.  The  best  method  of  preserving  timber  is  by  natural 
seasoning;  this  is  done  by  exposing  the  timber  to  air  in  a  dry 
place,  but  protected  from  hot  suns  or  high  winds.  This  is  a  slow 
process  and  requires  many  years  ;  the  time  may  be  lessened 
by  first  soaking  the  timber  in  water,  as  this  will  dissolve  a  por¬ 
tion  of  the  sap,  and  the  drying  will  be  more  rapid.  In  artificial 
seasoning  the  timber  is  exposed  in  a  close  building  to  a  current 
of  hot  air,  the  time  required  depending  upon  the  dimensions 
of  the  timber.  In  both  cases  the  timber  should  be  piled 
with  such  intervals  as  will  allow  free  contact  of  the  air  with  its 
entire  surface,  the  object  of  all  seasoning  being  to  extract  the 
sap  from  the  pores  of  the  timber.  The  immense  demand  for 
timber  has  practically  rendered  natural  seasoning  impossible, 
and  nearly  all  timber  used  in  floors,  doors,  window-frames, 
mouldings  of  houses,  etc.,  is  artificially  seasoned.  But  for  the 
large  structures  on  railroads,  and  similar  structures,  the  timber 
has  little  or  no  seasoning,  and  where  great  durability  is  required 
some  artificial  method  of  preserving  the  timber  must  be  re¬ 
sorted  to.  But  it  is  useless  to  resort  to  any  of  these  means 
unless  the  sap  is  first  removed,  as  the  object  of  seasoning  is  to 
remove  the  sap,  so  the  mode  of  preserving  the  timber  is  to 
keep  the  moisture  from  again  entering  the  pores,  by  filling 
them  either  entirely  or  partly  at  and  near  the  exposed  surfaces 
with  a  durable  and  impervious  substance. 

45.  For  wooden  bridges  a  very  excellent  method  is  to  en¬ 
tirely  enclose  the  bridge  in  a  sheeting  of  plank  and  a  tin  or 


l84  a  practical  treatise  on  foundations. 

shingle  roof ;  by  this  means  the  timber  has  time  to  be  naturally 
seasoned  at  the  same  time  that  the  structure  is  being  used.  The 
timbei  in  such  cases  will  last  for  many  years ;  cracks  will  be 
prevented,  and  if  at  the  expiration  of  one  or  two  years  the 
timbers  of  the  bridge  are  painted  and  kept  well  covered  with 
paint,  the  structure  will  last  for  a  great  length  of  time.  But  the 
piauice  of  painting  timber  before  the  sap  is  entirely  removed 
is  to  be  condemned,  as  it  does  more  harm  than  good  ;  it  pre¬ 
serves  the  timber  from  decay  on  the  surface,  but  hastens  the 
rot  on  the  inside.  Builders  of  wooden  bridges  are  always  in  a 
hurry  to  paint  the  timbers,  as  it  tends  to  prevent  cracks  before 
the  structure  is  completed  ;  but  cracks  are  the  lesser  evil  of  the 
two.  Timbers  of  bridges  should  not  be  painted  when  fresh  from 
the  mills.  The  red  paints  are  almost  exclusively  used;  but 
these  differ  materially  in  durability,  and  only  the  standard 
brands  should  be  used.  Oil  paints  in  any  colors  can  now  be 
put  chased  in  cans  or  barrels,  and  ready-mixed  for  use.  A  coat¬ 
ing  of  pitch  or  tar  is  also  used  as  a  paint ;  it  is  unsightly,  but 
gives  good  results. 

46.  Other  artificial  means  of  preserving  timber  consist  in 
filling  the  pores  of  the  timber  with  solutions  of  metallic  salts, 
such  as  copperas  or  sulphate  of  iron,  corrosive  sublimate  or 
bichloride  of  mercury,  chloride  of  zinc,  sulphate  of  copper. 
The  timber  can  be  saturated  with  these  salts  after  the  sap  has 
been  expelled,  or  forcing  them  into  the  pores  under  pressure, 
diiving  the  sap  ahead.  All  of  these  substances  seem  to  pre¬ 
serve  the  timber  as  long  as  they  remain  in  the  pores ;  but  they 
are  gradually  dissolved  and  removed  by  water.  Owing  to  the 
expense  and  time  required  in  the  application  of  these  they  are 
but  little  used  in  this  country.  Owing  to  the  abundance  and 
cheapness  of  good  timber,  it  is  found  more  economical  to  let 
the  timber  rot  and  renew  the  structure  from  time  to  time. 
Creosote,  or  the  heavy  oil  of  tar,  has,  however,  been  used  to 
a  considerable  extent  in  this  country,  and  it  is  claimed  that, 
although  it  will  materially  increase  the  first  cost  of  the  struc¬ 
ture,  it  will  ultimately  prove  economical,  as  it  adds  greatly  to 
the  durability  of  the  timber,  which  lasts  three  or  four  times  as 


MEANS  OF  PRESERVING  TIMBER.  1 85 

long  as  timber  not  creosoted.  The  sap  and  moisture  are  first 
exhausted  by  creating  a  partial  vacuum  in  an  air-tight  vessel  or 
tank,  and  then  forcing  the  creosote  into  the  pores  of  the  timber 
under  a  heavy  pressure.  This  is  a  rather  expensive  process, 
making  the  cost  of  the  structure  from  2  to  2^  times  as  much 
as  the  natural  timber,  and  for  the  reason  above  stated  it  is  used 
to  only  a  limited  extent.  If  it  were  universally  used,  the  cost 
might  be  materially  reduced.  As  it  is,  the  application  is  mainly 
confined  to  the  treatment  of  piles  and  timber  used  in  sea-water; 
here  it  becomes  a  necessity,  as  timber  is  rapidly  destroyed  by 
sea-worms.  The  Teredo  navalis  enters  the  timber  no  larger 
than  the  point  of  a  pin,  and  eats  towards  the  centre,  growing 
as  it  progresses,  reaching  the  size  of  a  grub-worm.  Thousands 
of  these  worms  attack  the  timber,  completely  honeycombing 
it,  and  destroying  it  in  less  than  a  year.  It  is  difficult  to  detect 
the  infinitesimal  holes  on  the  surface.  Large  piles  have  to  be 
renewed  in  a  year  after  immersion.  These  worms,  however, 
only  eat  between  the  low-water  line  and  the  bed  of  the  stream. 
It  has,  therefore,  been  suggested  to  cover  the  piles  with  a 
sheeting  of  copper  or  copper  tacks,  or  to  fasten  a  hollow  cast- 
iron  cylinder  to  the  top  of  the  pile,  and  drive  the  pile  by  means 
of  blocks  of  wood  resting  on  top  of  the  iron,  to  receive  the 
blow  of  the  hammer,  or  to  place  a  follower,  a  short  pile  in  the 
cylinder,  resting  on  top  of  the  pile,  and  reaching  above  the  cyl¬ 
inder  in  order  to  transmit  the  effect  of  the  blow.  Iron,  however, 
stands  but  poorly  the  corrosive  effect  of  sea-water ;  none  of 
these  have  yet  been  proved  to  give  economical  and  satisfactory 
results.  Creosote  is,  therefore,  commonly  resorted  to.* 

47.  The  durability  of  timber  is  materially  affected  by  the 
time  of  cutting  down  the  trees  ;  this  should  be  done  generally 
in  the  winter,  when  the  sap  is  not  running.  Ordinarily,  little 
attention  is  paid  to  this,  and  the  trees  are  cut  down  and  carried 
to  the  mill  when  needed,  regardless  of  the  time  of  the  year. 

*  So-called  “vulcanized  timber”  is  now  produced  by  subjecting  green 
timber  to  a  great  pressure  and  high  temperature,  which  converts  the  sap  and 
other  deleterious  fluids  into  antiseptic  compounds,  which  become  more  or  less 
solid  and  impervious  to  water.  Experiments  indicate  an  increased  strength, 
stiffness  and  durability. 


1 86  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 

The  age  at  which  trees  are  cut  is  also  a  matter  of  much  impor¬ 
tance.  Owners  of  forests  desire  to  get  rid  of  the  older  trees,, 
and  timber  from  trees  that  show  evident  signs  of  deterioration 
from  age  is  often  forced  on  the  engineer.  Such  timber  is  brittle, 
sometimes  soft,  spongy  and  “  doty  the  latter  can  be  easily 
detected  by  its  appearance,  and  it  will  clog  the  saw  while  cutting 
through  it  ;  such  timber  has  really  commenced  to  decay,  and 
will  rapidly  rot  when  exposed.  Often  a  stick  of  timber  will 
show  this  “dotiness”  at  one  end  ;  the  other  end  may  be  per¬ 
fectly  sound,  apparently,  and  even  cutting  off  a  foot  or  two,  the 
entire  stick  may  seem  sound  and  have  a  better  appearance 
than  many  other  pieces  It  is  hard  to  condemn  such  timber, 
and  it  is  often,  if  not  generally  used.  It  is  a  safe  rule,  however, 
to  condemn,  as  it  evidently  indicates  a  defective  tree. 

48.  All  kinds  of  timber  constantly  immersed  in  water  will 
last  indefinitely.  Timber  exposed  to  alternate  moisture  and 
dryness  will  decay  rapidly,  as  also  when  moist,  and  in  a  warm 
or  confined  air.  Timber  exposed  to  confined  air  alone  decays 
by  what  is  known  as  “  dry  rot,”  and  crumbles  into  a  powder. 

49.  As  before  mentioned,  in  exposed  structures,  such  as 
trestles,  the  timber  rots  first  at  certain  well-determined  points, 
as  the  joints  in  framed  structures,  and  while  it  is  expen¬ 
sive  and  impracticable  to  undertake  to  preserve  the  timber  of 
such  structures  by  any  of  the  processes  above  mentioned,  it  is 
easy  and  comparatively  inexpensive  to  prolong  the  life  of  these 
timbers  at  the  joints,  and  thereby  the  life  of  the  entire  struc¬ 
ture,  as  the  strength  and  durability  of  the  weakest  part 
measures  that  of  the  whole  structure.  This  is  done  simply  by 
painting  all  surfaces  of  contact  between  timbers,  as  the  top  and 
bottom  of  every  post,  the  ends  of  the  stringers,  with  hot  asphalt 
or  coal-tar,  either  alone  or  properly  thickened  with  lime,  applied 
just  before  putting  the  parts  together;  this  is  simple,  cheap, 
easily  and  rapidly  applied.  To  include  this  item  in  the  contract 
would  cause  an  exaggerated  demand  for  increase  in  price.  It 
would  be  carelessly  executed  and  often  neglected  altogether 
but  even  under  these  circumstances  it  would  be  a  great  im¬ 
provement  on  the  ordinary  plan.  But  it  would  pay  the  com- 


MEANS  OF  PRESERVING  TIMBER.  1 8/ 

pany  to  employ  a  man  whose  entire  time  should  be  devoted  to 
a  faithful  discharge  of  this  duty,  and  to  purchase  the  necessary 
materials.  Three  fourths  of  the  timber  removed  from  old 
structures  is  often  as  sound  as  when  first  used;  and,  if  this 
simple  remedy  will  preserve  the  remaining  one  fourth,  it  should 
certainly  be  adopted. 

Article  XXXIX. 

TIMBER  TRESTLES— (CONTINUED). 

50.  There  are  two  other  types  of  trestle  possessing  several 
advantages,  but  seem  to  be  seldom  used  ;  these  will  be  briefly 
described.  In  the  first,  whether  the  trestle  is  one  or  more 
stories  high,  the  posts  are  made  of  several  small  pieces  of 
timber  bolted  together,  instead  of  one  piece,  as  in  the  ordinary 
trestle.  Four  pieces,  6  ins.  X  6  ins.,  give  an  area  of  144  square 
inches,  the  same  as  one  piece  12  ins.  X  12  ins.  These  pieces 
bolted  together  with  packing  blocks  between  give  stiffness  to 
the  columns  by  increasing  the  ratio  of  the  least  diameter  to 
length  ;  cross  and  longitudinal  pieces  can  be  placed  between 
the  pieces,  making  thereby  in  some  respects  a  better  and 
stronger  connection  between  all  of  the  main  members  of  the 
structure,  and  with  an  increased  number  of  bolts;  pieces  and 
members  can  be  built  into  a  structure  possessing  great  stiffness 
and  strength.  This  form  of  trestle  has  been  used  in  very  high 
trestles  and  timber  piers.  The  columns  may  contain  any 
number  of  the  small  pieces  bolted  together,  depending  upon  the 
height  of  the  trestles  and  the  length  of  spans  to  be  carried. 

51.  The  other  type  differs  slightly  from  any  of  those  above 
described.  The  writer  has  never  seen  it  constructed,  but  it 
seems  to  possess  the  advantage  of  bringing  each  member  to  bear 
a  part  of  the  load  at  the  same  time  that  it  gives  spread  to  the 
base  and  increased  lateral  stiffness  ;  it  can  be  better  understood 
by  conceiving  each  story  first,  as  framed  for  a  single  story 
trestle ;  set  one  on  the  top  of  the  other,  and  each  successive 
story  to  have  one  additional  batter-post  on  each  side  of  the 
centre  parallel  to  the  batter-post  in  that  section,  but  placed  so 


1 88  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 

as  to  be  in  the  prolongation  of  the  batter-posts  in  the  sections 
above ;  the  caps  and  sills  of  each  section  being  lengthened  to 
suit  the  above  conditions,  the  usual  longitudinal  and  diago¬ 
nal  braces  being  used.  In  this  construction,  the  outside 
batter-posts  extend  in  one  continuous  line  from  top  to  bottom, 
the  next  from  the  bottom  of  the  first  to  the  bottom,  and  so  on. 
Distance  between  vertical  posts  gradually  and  uniformly  dimin¬ 
ishes  from  top  to  bottom.  This  avoids  a  sort  of  haphazard 
placing  of  the  additional  pieces  in  the  lower  sections,  which 
often  exists,  and  sometimes  looks  as  if  pieces  of  timber  were 
simply  inserted  to  fill  up  vacant  spaces,  without  any  regard  to 
their  forming  any  integral  part  of  the  structure  itself.  And  in 
fact,  the  strength  of  the  timbers  exceeds  so  many  times  the 
strains  brought  upon  them  that  not  much  pains  is  expended 
upon  placing  them  in  such  a  manner  as  to  be  of  any  great  ad¬ 
vantage.  (See  Figs.  I,  2,  3,  and  4,  Plate  VII.) 

52.  In  trestles  especially,  the  joints  are  of  importance,  as 
the  structure  is  generally  built  of  green  timber  entirely;  is 
exposed  to  all  conditions  of  weather  without  any  protection  of 
any  kind,  and  is  generally  run  over  at  high  speed,  causing 
constant  hammering  and  vibration,  if  at  all  loose.  The  repairs 
are  made  in  detail  and  constantly,  without  stopping  the  trains. 
One  piece  is  taken  out  at  a  time  and  a  new  piece  inserted. 
Some  forms  of  joints  are  more  liable  to  rot  than  others  ;  some 
reduce  the  effective  bearing  surfaces  more  than  others  ;  some 
make  it  more  difficult  to  remove  a  piece  than  others.  There¬ 
fore  that  form  of  joint  which  offers  the  least  number  of  the 
above  objections,  due  regard  being  had  to  the  strength  of  the 
structure,  should  be  adopted. 

Article  XL. 

JOINTS  AND  FASTENINGS. 

53.  The  principal  joint  in  trestles  connects  a  strut  and  a  tie 
or  a  strut  and  a  beam.  As  a  general  rule,  the  mortise-and-tenon 
joint  is  used.  (See  Figs.  1,  5,  Plate  VIII.)  This  joint  is  formed 
by  cutting  a  rectangular  hole  in  the  face  of  one  of  the  timbers, 
about  8  or  8-^  ins.  long,  3  to  3^  ins.  broad,  and  about  6  ins. 


JOINTS  AND  FASTENINGS.  1S9 

deep,  and  cutting  on  the  end  of  the  other  piece  a  tenon,  theo¬ 
retically  of  the  above  dimensions,  but  practically  from  |  to  -A-  in. 
smaller  in  each  dimension.  This  tenon  is  inserted  into  the  mor¬ 
tise,  and  in  a  hole  from  to  iA  ins.  bored  through  the  tenon 
an  oak  pin  or  treenail  is  driven ;  this  constitutes  the  connec¬ 
tion.  In  this  joint  from  24  to  27  sq.  ins.  of  bearing  surface  are 
lost  at  the  centre  of  the  post,  as  the  tenon  never  fills  accurately 
and  fully  the  mortise.  The  bearing  surface  of  the  post  con¬ 
sists  in  a  narrow  shoulder  around  the  tenon.  Not  unfrequently 
the  post  has  sap  on  the  corners,  extending  often  on  the  face  of 
timber  for  several  inches,  practically  reducing  the  bearing  sur¬ 
face  in  proportion  to  the  amount  of  sap.  The  mortise  forms  a 
receptacle  for  water ;  sometimes  a  hole  is  bored  in  the  bottom 
of  the  mortise  entirely  through  the  timber.  This  prevents  any 
great  accumulation  of  water,  and  affords  also  slight  ventilation  ; 
but  nevertheless  water  enters,  and  remains  for  a  greater  or  less 
time.  The  tenon  only  serves  to  hold  the  pieces  together. 
This  joint,  therefore,  reduces  the  strength  of  the  pieces,  in¬ 
creases  the  tendency  and  rapidity  of  decay,  the  strength  of 
the  joint  itself  depending  upon  a  small  projection  of  timber  in 
conditions  most  favorable  for  rot.  Yet  it  is  used  more  than 
any  other  joint.  The  hot  tar  and  lime  paint  would  certainly  be 
of  very  great  advantage  in  this  case.  It  is  difficult  to  insert  a 
new  timber,  when  a  rotten  one  is  removed,  without  lifting  the 
entire  cap.  The  cost  of  framing  is  somewhat  increased  in 
cutting  the  mortise-and-tenon. 

54.  The  writer  has  for  the  above  reasons  used  to  a  great 
extent  the  joint  shown  in  Fig.  2,  Plate  VIII.  This  joint  is 
formed  by  simply  cutting  a  notch  or  dap  about  1  in.  deep  entirely 
across  one  of  the  pieces;  the  end  of  the  other  is  simply  cut 
square,  fitting  into  the  dap.  By  this  means  the  entire  strength 
of  both  pieces  is  available,  moisture  is  less  apt  to  enter  and 
remain,  a  new  piece  is  more  readily  inserted.  The  rotting  of 
the  sap  on  the  outside  of  the  timber  affects  the  strength  of  the 
post  but  little  in  proportion,  as  the  harder  and  stronger  heart 
remains.  The  post  can  be  slightly  chamfered  at  the  top,  so 
that  the  cap  can  slightly  project  over  it,  materially  aiding  in 


190  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 

excluding  the  water.  The  pieces  are  held  together  either  by 
drift-bolts,  or  preferably  by  straps  and  bolts,  drift-bolts  render* 
ing  repairs  very  difficult.  Timber  strips  can  be  used  instead 
of  iron  straps,  but  this  increases  the  tendency  to  rot,  and  makes 
a  bungling-looking  joint.  It  would  seem  that  this  joint  would 
possess  in  every  respect  a  great  advantage  over  the  mortise- 
and-tenon  joint. 

55.  A  dovetail-joint,  or  a  joint  formed  by  halving  the  tim¬ 
bers  into  each  other,  as  in  Fig.  3,  Plate  VIII,  and  bolted  to¬ 
gether,  forms  a  good  connection,  but  is  rarely  used  in  trestle 
construction  ;  yet  it  is  the  usual  joint  used  where  both  of  the 
pieces  are  horizontal,  particularly  in  cribs  for  holding  concrete 
or  broken  stone,  and  in  the  walls  of  crib  coffer-dams,  as  the 
pieces  act  both  as  a  strut  and  tie  brace. 

56.  The  caps  and  sills  are  sometimes  made  in  two  pieces 
6  ins.  X  12  ins.,  instead  of  one  piece  12  ins.  X  12  ins.;  these 
pieces  placed  an  inch  or  an  inch  and  a  half  apart ;  daps  of  suf¬ 
ficient  depth  cut  into  these,  so  that  the  tenon  can  enter  between 
them ;  the  pieces  bolted  together  between  the  posts  and  also 
through  the  tenon.  This  possesses  the  following  advantages : 
The  tenon  has  good  ventilation  and  does  not  rot  so  rapidly; 
the  cap  and  sill  are  likewise  preserved;  new  caps,  sills,  and 
posts  can  be  inserted  with  ease. 

57.  The  inclined  or  batter  posts  of  trestles  are  connected 
in  the  same  manner  as  the  vertical  post.  When  the  mortise- 
and-tenon  joint  is  used,  it  is  so  cut  that  the  tenon  stands  in  a 
vertical  direction  when  connected,  and  not  in  the  prolongation 
of  the  axis  of  the  piece,  as  seen  in  Fig.  5,  Plate  VIII;  or  it 
can  be  made  as  seen  in  Fig.  6,  Plate  VIII.  In  these  cases  the 
tenon  or  shoulder  should  be  at  least  2  ft.  from  the  end  of  the 
piece  in  the  direction  of  the  strain,  so  as  to  prevent  shearing  off 
the  timber.  If  the  posts  are  very  much  inclined,  this  tendency 
should  be  resisted  by  bolts  as  shown  in  Figs.  6  and  8,  Plate 
VIII.  This  is  rarely  ever  necessary  or  used  in  trestle  construc¬ 
tion. 

58.  Longitudinal  bracing  is  not  generally  used  in  a  single¬ 
story  trestle,  unless  the  trestle  is  quite  high  ;  but  is  always  used 


JOINTS  AND  FASTENINGS.  I9I 

in  a  trestle  of  two  or  more  stories.  It  is  a  good  and  safe  rule, 
however,  to  use  it  whenever  the  trestle  is  over  10  or  12  ft.  high. 
A  continuous  course  of  3-in.  plank,  spiked  or  bolted  to  the  out¬ 
side  posts,  will  add  greatly  to  the  stiffness  of  the  trestle,  and 
should  always  be  used  if  the  trestle  is  constructed  on  a  grade 
or  incline.  If  spiked  to  the  top  of  one  bent  and  the  bottom  of 
the  next  in  the  direction  of  the  descending  grade,  it  will  be 
more  effective  than  if  placed  horizontally.  For  higher  trestles 
it  is  6  X  6  in.  or  8  in.  timber,  and  is  bolted  to  all  posts,  at  the 
top  of  each  section.  Diagonal  longitudinal  bracing  is  also 
sometimes  used.  The  horizontal  bracing  at  the  division  be¬ 
tween  the  sections  or  stories  is  generally  notched  over  the 
caps  as  well  as  bolted  to  them. 

59.  Besides  the  above-mentioned  joints,  which  are  principally 
used  in  connecting  pieces  that  make  either  acute  or  right 
angles  with  each  other,  there  are  others  for  lengthening  ties  or 
struts.  In  the  case  of  struts  which  transmit  a  compressive 
strain,  all  that  is  necessary  for  this  purpose  is  to  bring  the  ends 
square  together.  But  unless  fastened  in  position,  there  is  dan¬ 
ger  of  one  piece  slipping  on  the  other.  This  can  be  prevented 
by  the  simple  fish-joint,  which  consists  in  spiking  or  bolting 
strips  of  wood  or  iron  straps  to  the  faces  of  the  pieces  (see 
Figs.  12,  13,  and  14),  or  by  halving  the  pieces  together  and  hold¬ 
ing  them  in  position  by  bolts  or  straps,  or  by  a  combination  of 
these  (see  Figs.  10  and  14,  Plate  VIII).  When  the  pieces  overlap 
and  are  bolted  together  the  joint  is  called  a  scarf  joint.  Any 
of  these  joints  shown  in  the  above-mentioned  drawings  will 
serve  to  lengthen  struts.  But  as  the  strain  on  the  bolts,  straps, 
or  fish-plates  is  small,  the  simplest  connection  is  the  best  and 
the  cheapest.  Iron  sockets  are  used  also  in  many  cases,  the  ends 
of  the  pieces  being  simply  inserted  into  the  open  space,  and  may 
or  may  not  be  bolted ;  or  holes  may  be  bored  into  the  ends 
of  the  pieces,  and  an  iron  pin  inserted  in  one  piece  and  pro¬ 
jecting  out,  the  other  piece  being  then  placed  on  top.  This 
makes  a  rather  weak  connection  if  there  is  any  tendency  to 
bend.  Round  timbers,  such  as  piles,  can  be  lengthened  by 
halving  the  pieces,  and  when  placed  together  drive  iron  rings 


1 92  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 

on  tightly  so  as  to  bind  or  clasp  the  pieces  together.  This  is 
perhaps  the  best  method  of  splicing  piles. 

60.  To  join  ties  together,  any  of  the  joints  (Figs,  io,  II,  12, 
13,  and  14,  Plate  VIII)  answers  the  purpose  ;  that  is,  either  the 
fish  or  scarf  joint,  or  both.  It  must  be  remembered  in  this 
joint  that  as  the  tendency  is  to  pull  the  pieces  apart,  the  con¬ 
nections  or  fastenings  have  the  entire  strain  to  bear,  and  should 
therefore  be  as  strong  as  the  parts  connected,  after  deducting 
for  bolt-holes,  indents,  etc.  We  will  describe  each  joint  briefly. 
Fig.  12  is  a  simple  fish-joint.  The  aggregate  strength  of  the  fish¬ 
plates  A  must  be  the  same  as  that  of  the  uncut  pieces;  their 
length  must  be  such  that  when  under  maximum  tension  the 
bolts  must  not  shear  or  cut  out  to  the  ends  of  either  the  fish¬ 
plates  or  the  main  members  ;  or  the  area  sheared  or  split,  multi¬ 
plied  by  the  resistance  to  shearing  must,  be  equal  to  the  area  of 
the  ties  multiplied  by  the  resistance  to  tearing,  also  to  the  sum 
of  the  areas  of  all  the  bolts  multiplied  by  the  resistance  of 
wrought-iron  to  shearing  across  ;  or  in  symbols,  F  X  s  —  T  X  t 
=  B  X  i,  in  which  F  =  equal  area  of  timber  liable  to  shearing, 
s  =  safe  resistance  to  shearing  per  unit  of  area  ;  T  =  to  effec¬ 
tive  cross-section  of  ties,  i.e.,  after  deducting  bolt-holes,  indents, 
etc.;  /  =  safe  tensile  strength  per  unit  of  area;  5  =  sum 
of  areas  of  bolts;  i  —  resistance  to  shearing  of  iron  per  unit  of 
area.  Assuming  unit  of  area  as  1  sq.  in.,  and  area  of  ties  as 
144  sq.  ins.  (12  X  12  in.  sticks),  and  substituting  the  average 
units  of  strength,  we  have  F  X  400  =  144  X  IOOO  =  B  X  40,000, 
hence  F  —  360  sq.  ins.,  or  in  a  stick  12  ins.  deep  a  single  bolt 
should  be  at  least  2.5  ft.  from  the  end  of  the  timbers,  or  two 
bolts  in  the  same  distance,  one  1.25  ft.  from  the  end;  and 
B  =  3.6  sq.  in.,  or  two  bolts  1^  in.  diameter.  This  gives  the 
least  allowable  values  for  F  and  B,  as  the  least  unit  values  for 
tensile  strength  and  the  greatest  unit  values  for  shearing,  both 
for  iron  and  timber,  is  used.  In  this  case  both  fish-plates  and 
bolts  have  the  entire  strain  to  bear,  each  on  its  own  account. 

61.  In  Fig.  13  the  fish-plates  are  indented  into  the  ties;  the 
effective  area  of  the  tie  is  therefore  reduced  to  that  extent,  and 
the  strength  of  the  connections  is  increased  as  additional  areas 


JOINTS  AND  FASTENINGS. 


193 


are  presented  to  resist  shearing  ;  therefore  the  connections  need 
not  be  as  strong  as  in  the  first  case.  The  principle  of  determin¬ 
ing  the  number  of  the  bolts  and  their  position,  as  also  area  of 
fish-plate  to  be  sheared,  are  the  same  as  above.  But  this  joint 
sacrifices  the  strength  of  the  main  tie,  and  causes  great  waste 
of  material.  Both  joints  present  a  bungling  appearance.  Iron 
bars  or  straps  answer  the  same  purpose  and  look  much  better, 
and  in  permanent  structures  are  ultimately  more  economical. 

62.  In  the  simple  scarf-joint,  Fig.  10,  the  strength  of  the 
joint  depends  entirely  upon  the  resistance  to  shearing  of  the 
timber  and  the  iron  bolts  ;  the  effective  area  of  the  tie  is  re¬ 
duced  to  one  half ;  consequently  50  per  cent  of  the  timber  is 
wasted,  in  addition  to  the  waste  in  the  overlap,  each  stick 
being  from  4  to  8  ft.  longer  than  actually  required  in  the  fis’n- 
joint.  In  these  joints  hardwood  keys  are  introduced,  as  shown 
in  the  drawings ;  these  being  driven  between  the  ties,  project¬ 
ing  an  inch  or  two  into  both,  increase  the  areas  to  be  sheared, 
the  bolts  serving  mainly  to  bring  the  parts  into  close  con¬ 
tact.  Fig.  14  shows  a  combination  scarf  and  iron  fish-joint, 
with  bolts  and  keys.  It  does  not  seem  to  possess  any  advan¬ 
tage  over  the  plain  fish-joint  with  iron  straps,  and  is  more 
difficult  to  frame.  Keys  should  always  be  placed  horizontally  ; 
if  vertical,  water  easily  enters,  and  causes  rot.  Fig.  1 1  shows  a 
scarf-joint  which  will  hold  without  bolts ;  only  one  third  of  the 
strength  of  the  timber  is  secured,  and  unless  the  timber  is 
thoroughly  seasoned,  and  the  framing  carefully  executed,  there 
is  but  little  strength  in  the  joint ;  bolts  should  always  be  used 
in  all  joints  to  resist  a  tensile  strain,  whether  fish  or  scarf 
joints. 

63.  Timber  has  a  greater  resistance  to  tearing  than  it  has 
to  crushing,  but  owing  to  the  difficulty  of  connecting  several 
pieces  to  resist  tensile  strain,  as  seen  above,  without  great  waste 
of  material,  the  general  practice  is  to  use  timber  for  members 
under  compressive  strain  and  iron  for  those  members  under 
tensile  strain,  unless  single  sticks  can  be  secured  of  sufficient 
length.  In  Howe-truss  bridges  for  railways,  and  timber  high¬ 
way  bridges,  the  bottom  chords  are  made  of  timber.  In  the 


194  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


first  iron  fish-bars  are  almost  always  used  at  the  joints,  in  the 
second  wooden  fish-plates  with  indents  or  projections:  and 
in  addition  the  chords  are  made  of  three  or  more  pieces  thor¬ 
oughly  bolted  together  at  short  intervals,  with  iron  or  wooden 
packing-blocks  between. 

64.  Figs.  8  and  9,  Plate  YI 1 1,  show  the  construction  of  king 
and  queen  post  roof  or  bridge  trusses  for  spans  from  20  to  40  ft. 
in  length.  The  lower  horizontal  member  is  called  the  tie-beam 
and  is  under  tension  ;  the  upper  is  the  straining  beam  and  is 
under  compression.  The  end  inclined  members  are  struts  under 
compression.  The  verticals  are  ties  under  tensile  strain  ;  the  one 
on  the  left  is  a  single  piece  resting  on  top  of  the  tie-beam  and 
connected  with  it  by  a  stirrup,  which  is  simply  an  iron  strap,  or 
bar  bent  at  right  angles  at  the  ends,  passed  up  under  the  tie- 
beam  and  held  by  bolts  to  the  vertical.  The  one  on  the  right  is 
made  in  two  pieces,  the  tie-beam  passing  between  them  and 
resting  on  shoulders  cut  in  them  from  1  to  2  ft.  from  the  lower 
end,  the  two  pieces  held  together  by  bolts,  packing-blocks 
being  placed  between  where  necessary.  This  is  an  over¬ 
trussed  or  through  span.  Fig.  3,  Plate  VII,  represents  the 
under-trussed  or  deck  span  constructed  for  the  same  purpose. 
This  is  used  in  trestle-work  when  the  spans  are  from  20  to  25 
ft.  from  bent  to  bent.  Instead  of  these  trussed  beams,  timber 
built  beams  are  sometimes  used.  Figs,  la,  2 a,  4 a,  5 a,  6a, 
Plate  VIII,  show  this  construction,  in  which  2,  3,  or  4  pieces 
of  timber  are  built  together  with  bolts  and  keys.  Two  pieces 
12X12  ins.,  one  on  the  top  of  the  other,  will  answer  for  spans 
from  12  to  15  ft.  long;  two  pieces  side  by  side,  with  a 
third  on  top,  for  spans  from  15  to  20  ft;  and  four  pieces 
for  spans  from  20  to  25  feet.  It  is  evident  that  there  is  a 
great  waste  of  timber,  and  it  is  badly  distributed  to  meet  the 
required  conditions  of  strain.  Single  pieces  properly  trussed 
would  be  more  sightly  and  more  economical.  Beams  are  built 
also,  either  as  straight  or  curved  beams,  of  plank  laid  flatwise 
and  bolted  or  spiked  together,  and  are  frequently  used  in 
bridges  and  roofs  ;  their  strength  is  materially  less  than  a  solid 
beam  of  the  same  cross-section,  probably  not  more  than  from 


JOINTS  AND  FASTENINGS. 


195 


one  third  to  two  thirds  as  strong.  As  a  rule,  they  would  not 
be  used  when  large  sticks  could  be  secured,  except  in  tempo¬ 
rary  structures,  such  as  centres  for  arches  (Figs.  7#,  8#,  9 a, 
Plate  VIII).  From  the  above  description,  there  are  certain 
general  principles  applicable  in  constructing  all  joints. 

1.  The  joints  should  be  so  cut,  and  the  fastenings  so  pro¬ 
portioned  and  placed,  as  to  weaken  the  main  members  as  little 
as  possible. 

2.  The  strength  of  the  fastenings,  bolts,  straps,  and  fish¬ 
plates,  either  singly  or  together,  should  be  equal  to  the  effective 
strength  of  the  parts  connected,  and  their  areas  should  be 
inversely  proportional  to  their  unit  strength. 

3.  All  surfaces  in  contact  should  fit  exactly  throughout,  and 
the  direction  of  the  planes  of  such  surfaces  should  be  perpen¬ 
dicular  to  the  direction  of  strains,  and  their  areas  should  be  suffi¬ 
ciently  large  to  keep  the  unit  strain  of  that  particular  kind  within 
safe  limits,  allowing  a  factor  of  safety  of  4  in  case  of  steady 
and  of  10  in  case  of  moving  loads. 

4.  All  joints  should  be  so  arranged  and  placed  as  to  pre¬ 
clude,  as  far  as  possible,  access  to  water,  and  should  be  painted 
with  some  substance  impervious  to  water. 

5.  The  joints  will  in  general  be  the  weakest  part  of  the 
structure,  and  should  be  taken  as  the  measure  of  the  strength 
of  the  whole. 

6.  In  all  built  or  packed  beams  the  parts  composing  the 
beam  should  break  joints  as  far  as  possible. 

7.  Where  joints  are  formed  by  indents,  shoulders,  or  tenons, 
and  held  together  by  bolts  or  straps,  each  should  have  suffi¬ 
cient  strength  to  transmit  the  entire  strain,  as  from  shrinking 
of  timber,  loosening  of  bolts,  or  other  causes,  either  may  have 
to  bear  the  entire  strain.  This  is  applicable  in  all  cases,  except 
in  lengthening  struts,  unless  constant  inspection  is  made  to 
see  that  all  parts  are  properly  adjusted. 

8.  Although  there  is  always  a  consideration,  admissible, 
of  the  frictional  resistances  between  surfaces  in  contact,  this 
should  not  be  relied  upon  to  any  but  a  very  limited  extent,  if 
at  all. 


196  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


Article  XLI. 

SUPPORTS  FOR  TRESTLES. 

65-  Trestles  are  supported  in  three  ways:  1.  By  mud-sills  ; 
2.  By  masonry  pedestals  ;  3.  By  piles. 

If  any  pretext  can  be  offered  the  first  is  always  adopted  ;  they 
consist  of  from  4  to  6  pieces  of  timber,  of  any  size  convenient, 
generally  4  or  5  ft.  long,  placed  under  the  bottom  sill,  either 
wholly  or  partly  imbedded  in  the  ground,  shallow  trenches  be¬ 
ing  excavated  to  receive  them.  With  no  precautions  taken  to 
provide  drainage,  and  regardless  of  the  nature  of  the  soil,  nat¬ 
urally  water  collects  in  the  trenches,  converts  the  soil  into  mud, 
the  motion  of  the  train  produces  a  churning  action,  the  trestle 
rises  and  falls,  gets  out  of  line  and  level,  is  then  adjusted  by 
driving  shingles,  thin  strips  of  plank,  or  anything  that  can  be 
procured  under  the  sills,  and  this  is  repeated  until  these  strips  or 
shims  are  piled  one  on  the  top  of  the  other  for  6  or  8  ins.  or  more, 
sometimes  only  under  one  or  two  of  the  posts.  Under  such 
circumstances  the  wonder  is  that  accidents  are  not  many  times 
more  frequent  than  they  are.  Occasionally  engineers  require  a 
double  row  of  sills  to  be  laid  first  and  the  mud-sills  placed  across 
and  at  right  angles  to  these.  This  serves  several  purposes  :  it 
may  place  the  foundation-bed  below  the  injurious  action  of 
frost  ;  it  increases  the  area  of  bearing  surface;  it  lifts  the  sill 
slightly  above  ground,  permitting  ventilation  and  consequently 
preserving  the  timber  ;  it  is  certainly  not  very  expensive.  It 
should  always  be  required. 

2.  When  the  nature  of  the  ground  will  admit  of  mud-sill 
under  trestles,  it  will  generally  be  found  that  rubble-stone  can 
be  obtained  at  a  small  cost.  Small  rubble  pedestals  should 
then  be  built,  in  pits  from  1^  to  2  ft.  deep  and  extending  from 
6  ins.  to  1  ft.  above  ground.  A  single  pedestal  2  to  2%  ft.  square 
under  each  post  will  be  sufficient.  This  is  as  much  superior  to 
the  last  method  as  that  is  to  the  simple  mud-sill. 

3.  Piles  are  used  when  the  ground  is  very  soft  for  any  great 


JOINTS  AND  FASTENINGS. 


19  7 


depth  below  the  surface,  or  in  swamps.  These  can  be  cut  off 
below  the  moisture  line  or  above  the  surface.  One  pile  is  driven 
so  as  to  be  under  each  post  of  the  trestle  ;  the  sill  rests  on  the 
piles,  and  is  fastened  to  them  by  mortise  and  tenon,  by  drift- 
bolts,  or  by  straps.  Commonly  where  piles  are  necessary,  the 
trestles  are  comparatively  of  small  height,  the  piles  reach  well 
above  ground,  and  the  stringers  rest  directly  upon  them.  In 
such  cases  the  structure  becomes  distinctively  a  pile  trestle,  as 
distinguished  from  a  framed  trestle,  and  will  be  explained 
under  that  head. 

66.  Framed  trestles  may  be  divided  into  four  classes  or 
types  presenting  some  marked  differences  in  construction. 

1.  That  in  which  the  principal  members  are  vertical,  except 
the  outside  batter-posts,  inclined  braces  of  smaller  cross-section 
being  introduced  to  give  steadiness  and  stiffness.  This  is 
probably  more  generally  used  than  any  other.  Fig.  3,  Plate  VI, 
represents  a  single  story  or  section. 

2.  The  M  trestle,  in  which  all  of  the  main  members  are 
inclined,  and  verticals  introduced  only  in  the  very  high  trestles, 
auxiliary  inclined  braces  of  smaller  cross-section  being  also  used. 
See  Fig.  4,  Plate  VI,  for  single  story,  and  Figs.  1,  2,  Plate  IX, 
for  two  or  more  stories,  with  details  of  important  parts. 

3.  The  trestle  in  which  the  columns  are  built  up  of  pieces 
of  small  cross-section,  instead  of  single  pieces  of  larger  cross- 
section.  It  can  be  put  together  as  in  either  of  the  above 
types. 

4.  That  form  in  which  two  vertical  columns  extend  from 
top  to  bottom,  inclined  members  are  used  to  bear  a  portion 
of  the  load,  and  to  act  as  main  members  and  braces  at  the  same 
time.  See  Figs.  1,  2,  3,  4,  Plate  VII.  This  form  of  trestle  has 
never  been  used  to  the  writer’s  knowledge,  but  it  certainly 
seems  to  be  as  strong  as,  if  not  stronger  than,  the  other  trestles 
in  common  use.  It  will  be  noticed  that  straps  are  used,  instead 
of  the  mortise-and-tenon  joints.  The  writer  believes  that 
they  make  a  better  and  stronger  trestle.  It  is  also  shown 
as  resting  on  masonry  pedestals  for  the  same  reason. 
Either  of  the  above  forms  of  trestle  is  strong  enough,  and 


198 


A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


that  one  should  be  adopted  which  requires  the  least  material, 
requires  the  least  amount  of  work  in  framing  and  erecting,  and 
is  more  easily  renewed  and  repaired  in  whole  or  in  part. 
Fig.  2,  Plate  VII,  shows  the  method  of  trussing  the  stringers  by 
the  use  of  straining  pieces  and  struts,  and  Fig.  3  by  the  use  of 
iron  rods.  In  the  latter  case  it  is  better  to  use  two  ties  or  rods, 
and,  to  prevent  the  necessity  of  boring  holes  in  the  string-pieces, 
the  stringer  should  be  made  of  three  pieces,  with  small  inter¬ 
vals  between  them  through  which  the  rods  can  pass.  On  the 
ends  of  the  stringers  thick  iron  washers  should  be  placed, 
through  which  the  rods  pass  and  upon  which  the  nuts  bear,  so 
as  to  prevent  the  rods  from  cutting  into  or  crushing  the  fibres 
of  the  timber  at  the  ends  of  the  stringers.  As  the  spans  are  25 
feet  from  bent  to  bent,  the  stringers  do  not  break  joint  on  the 
cap,  and  unless  bolsters  or  fish-plates  are  used,  the  stringers 
would  not  have  over  5  inches  of  bearing  on  the  caps,  which  is 
not  enough.  In  Fig.  2,  Plate  VII,  two  6-inch  fish-plates  are 
bolted  to  the  stringers  and  rest  on  the  cap ;  by  this  means 
the  bearing  surface  is  increased  and  at  the  same  time  the 
stringers  are  tied  together.  This  the  writer  prefers  to  the 
bolster  connection  as  shown  in  Fig.  3  under  the  stringer  and 
bolted  to  the  cap. 

67.  All  of  the  main  members  of  the  trestle-bent  are  sub¬ 
jected  to  a  compressive  or  crushing  strain,  except  the  bottom 
and  intermediate  horizontal  pieces  or  sills  ;  these  are  subjected 
to  a  tensile  strain  and  a  crushing  strain  across  the  grain  by  the 
pressure  on  the  posts.  The  tensile  strain  tends  to  shear  the 
layer  of  timber  between  lower  end  of  each  post  and  the  end  of 
the  sill.  To  resist  this  the  end  of  the  post  should  be  from  1^  to 
2  ft.  from  the  end  of  the  sill,  as  shown  in  all  of  the  drawings. 
As  the  inclination  of  the  batter  posts  is  small,  this  strain  is 
small,  and  consequently  bolts  are  not  needed  or  used.  The 
strength  of  the  timber  to  crushing  transversely,  whether  pine 
or  oak,  is  very  great ;  the  greatest  loads  that  can  possibly 
come  on  the  posts  would  make  no  impression  on  the  timber  of 
the  sills  between  them.  Trautwine  says  that  a  pressure  of 
1000  lbs.  per  square  inch  will  not  indent  yellow  pine  or  oak 


JOINTS  AND  FASTENINGS. 


I99 


more  than  the  thickness  of  a  sheet  of  writing  paper,  and  white 
pine  not  over  ^  of  an  inch.  The  writer  has  subjected  soft 
pine  cushions  in  testing  the  crushing  strength  of  stone,  to  a 
pressure  of  5000  lbs.  per  square  inch,  with  no  perceptible  effect 
upon  them.  The  upper  cap  is  under  both  longitudinal  and 
transverse  crushing  strain,  and  always  has  ample  strength. 
The  posts  are  under  longitudinal  compression.  The  lengths  of 
such  pieces  in  proportion  to  their  least  dimension  is  an  impor¬ 
tant  factor  in  their  strength.  In  very  short  columns  the  resist¬ 
ance  to  crushing  is  simply  proportional  to  the  area  of  the  cross- 
section. 

68.  The  coefficient  of  resistance  to  crushing,  or  the  strength 
per  square  inch  of  area,  for  green  timber  is  about  5000  lbs.  per 
square  inch.  This,  however,  decreases  in  a  rapid  ratio  as  the 
length  increases,  and  when  the  length  is  30  times  the  diameter 
or  least  side  it  would  crush  under  less  than  \  of  5000  —  2500 
lbs.  per  square  inch,  and  the  safe  load  should  not  exceed  ^  to 
■fa  of  this,  or  from  500  to  250  lbs.  Seasoned  timber  is  about 
twice  as  strong  as  the  green  timber.  The  usual  practice  is  that 
the  length  of  the  column  should  not  exceed  20  times  its  least 
dimension,  or  a  stick  12  X  12  ins.  should  not  be  more  than  20 
ft.  long;  so  we  find  that  the  height  of  a  story  or  section  of  a 
trestle-bent  does  not  exceed  20  to  25  ft.  If  the  bents  are  12^  ft. 
apart,  the  greatest  load  per  foot  of  span  would  be  about  6000 
lbs.  or  75,000  lbs.  on  each  bent  of  the  trestle.  This  is  really  sup¬ 
ported  by  four  pieces,  but  two  pieces  12  X  12  in.  =  144  square 
inches  would  bear  safely,  at  the  low  limit  of  250  lbs.  per  square 
inch,  72,000  lbs. ;  but  assume  that  the  two  batter  posts  together 
bear  of  the  load  and  the  two  vertical  posts  f  of  the  load, 
each  vertical  post  would  carry  only  25,000  lbs.,  and  each  batter 
post  12,500  lbs.  A  X  250  =:  25,000,  and  A'  X  250=  12,500;  or 
the  area  of  the  vertical  posts  =  A  =  100  sq.  in.;  or  one  piece  10 
X  loins,  or  12  X  8J  ins.  would  be  sufficiently  large  for  the  verti¬ 
cal  posts,  and  for  the  batter  posts  A'  =  50  sq.  ins.  or  1  piece 
6£  X  8  ins.,  and  in  the  M  trestle  each  post  would  carry  \ 


X  75,ooo  =  18,750  lbs.  A  — 


_  18750 


250 


75  sq.  ins.  and  each  post 


200 


A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


would  be  8  X  9a  or  8£  X  8£  ins.;  but  in  either  case  the  dimen¬ 
sions  are  seldom  if  ever  less  than  io  X  12  ins.,  and  more  com¬ 
monly  12  X  12  ins.  in  cross-section. 

69.  In  the  case  of  a  25-ft.  span  the  uniform  load  per  foot 
of  length  would  not  exceed  5000  lbs.,  or  on  each  bent  125,000. 
In  the  M  trestle  each  post  would  carry  31,250  lbs.  its  area  need 
not  exceed  125  square  inches  or  one  piece  12  X  io£  ins.;  and  for 
other  forms  of  trestle,  assuming  as  before  one  third  of  the  entire 
load  as  borne  by  the  batter-posts  and  two  thirds  by  the  vertical 
posts,  the  batter-posts  would  be  10X9  his.  each  and  the  verticals 
12  X  14  ins.  In  this  calculation  the  column  or  post  is  supposed 
to  be  at  least  30  ft.  long,  which  is  rarely  the  case,  the  diag¬ 
onal  or  X  bracing  increases  its  strength  materially;  so  it  will  be 
seen  that  even  in  25-ft.  spans  the  verticals  need  not  ^xceed  12  X 
12  ins.  In  very  high  trestles  the  pressure  on  the  lower  posts 
is  increased  by  the  weight  of  the  structure;  but  as  the  number 
of  posts  is  greater  in  the  lower  sections,  the  unit  pressure  on 
any  one  post  would  never  exceed  that  of  the  posts  in  the 
upper  story,  and  no  increase  in  the  area  of  the  posts  is  neces¬ 
sary  in  the  lower  sections  above  the  dimensions  given  above 
and  shown  in  the  drawings,  though  the  bottom  posts  are 
sometimes  12  X  14  ins. 

70.  To  determine  the  area  of  the  cross-section  of  the 
stringers  we  will  use  the  formula  mWl  =  nfb/i1,  as  the  beam 
is  under  a  transverse  load  of  6000  lbs.  per  lineal  foot,  and  is 
liable  to  give  way  by  bending  or  cross-breaking.  The  greatest 
possible  load  is  6000  lbs.  per  foot,  and  the  unsupported  length 
of  span,  being  in  bents  12^  ft.  centres,  only  ii-J  ft.  The  total 
uniform  load  is  69,000  lbs.,  and  the  equivalent  centre  load 

_  69,000  _  ^ijoo  lbs.;  and  as  this  is  supported  by  four  pieces 

6  in.  wide  or  thick  each,  each  piece  will  only  have  to  bear 

—  8625  lbs.  Then  we  have  in  =  W=  8625  ;  1  = 
4 

1 1.5  X  12  =  138  ins.  ;  n  —  ^  ;  f  —  IOOO ;  b  —  6  in.  Substituting 
and  finding  value  of  h,  we  have  \  X  8625  X  138  =  X  1000  X 
6  X  b'1 ;  .•.  h2  =  297.56  ;  .\  h  =  1  ins.,  the  depth  of  the  beam  ; 


JOINTS  AND  FASTENINGS. 


201 


from  this  each  stringer  should  be  composed  of  two  pieces 
6  X  I7|-  ins.  This  gives  a  little  greater  depth  than  the  actual 
practice,  on  account  of  the  large  factor  of  safety  used,  or  what 
is  the  same  thing,  the  small  value  of  f  and  the  large  value 
given  to  W.  The  actual  dimensions  in  practice  are  6  X  14  ins.  to 
O  X  15  ins.  For  longer  spans  the  calculation  in  every  respect 
would  be  similar,  but,  owing  to  the  difficulty  of  getting  good 
sound  sticks  over  15  to  16  ins.  deep,  the  usual  practice  is  when 
necessary  to  increase  the  number  of  stringers,  as  three  or  four 
pieces  6x15  ins.  But  for  spans  over  20  or  25  ft.  long  it  is 
best  to  truss  or  brace  the  stringers ;  this  trussing  is  equivalent 
to  dividing  the  length  of  the  span  into  three  parts,  as  shown 
in  Fig.  2,  Plate  VII,  or  into  two  parts,  as  in  Fig.  3,  Plate  VII. 
In  the  first  case  the  spans  are  only  about  8  ft.  long,  and  in  the 
second  12  ft.  in  a  25-foot  span  ;  so  it  is  evident  that  in  this 
case  no  increase  in  the  size  of  the  stringers  will  be  required. 
The  struts  in  the  bracing  will  have  to  bear  the  load  on  only  8  ft. 
of  span  or  48,000  lbs.,  and  being  four  in  number,  each  will 
have  12,000  lbs.,  or  at  250  lbs.  per  square  inch  there  will  be 
necessary  48  sq.  ins.  in  each  strut,  or  a  single  piece  6x3  in. 
In  the  second  case  the  tension-rods  under  each  stringer  will 
have  to  carry  a  load  on  12  ft.  of  span  or  72,000  lbs.  and  on 
each  stringer  36,000  lbs. ;  but  this  passes  from  the  middle 
through  the  rods  on  both  sides  to  the  end,  the  rod  has  only 
to  bear  18,000  lbs.  of  load.  This  produces  a  pull  or  tension  on 
the  rod  equal  to  the  load  multiplied  by  the  length  of  the  rod 

and  divided  by  its  vertical  reach,  or  18,000  X  —  =  90,000 

2.25 

lbs.  The  tensile  strength  of  iron  is  about  50,000  lbs.  per  square 
inch,  and  with  a  factor  of  safety  of  4,  12,500  lbs.,  there  results 

99,000  |  g  jns>  nearly,  or  a  single  rod  3  in.  diameter  or  2 


rods  2\  in.  diameter  each.  The  pull  on  these  rods  tends  to 
crush  the  end  of  the  stringer.  Large  washers  should  be  used  to 
keep  the  pressure  in  safe  limits.  An  iron  plate  12X6  in.  = 
72  sq.  ins.  would  reduce  the  unit  of  pressure  to  about  1400  lbs., 


202  A  PRACTICAL  TREATISE  OX  FOUNDATIONS. 

which  would  be  safe.  Increasing  the  length  of  the  vertical 
decreases  pull  on  the  rods. 

71.  The  writer  has  entered  into  considerable  detail,  as 
it  shows  the  principles  of  calculating  the  several  kinds  of 
strain,  determining  the  sizes  of  the  different  members  and  the 
proper  construction  and  connection  of  the  parts,  so  as  to 
keep  the  unit  strains  within  safe  limits,  and  are  equally  appli¬ 
cable  to  bridges,  trestles,  floors  of  warehouses,  etc.  The 
formula  above  used  is  general,  easily  remembered,  and  easily 
applied  when  the  principle  of  the  lever  or  moments  is  under¬ 
stood.  The  amount  and  distribution  of  the  load  and  the 
way  in  which  the  beam  is  supported  are  known.  In  the 
formula  mWl  —  nfbh~,  m  is  a  constant  depending  upon  the 
distribution  of  the  load  and  the  manner  of  supporting  the 
beam  ;  W  is  the  total  load  on  the  beam  ;  l  is  the  clear  span  or 
unsupported  length  of  the  beam  ;  n  varies  with  the  shape  of 
the  beam,  and  for  beams  of  square  or  rectangular  cross-section 
is  equal  to  £  ;  b  —  breadth  of  beam  ;  h  —  depth, — all  dimensions 
in  inches;  /The  modulus  of  rupture,  which  varies  from  10,000 
to  5000  lbs. ;  but  only  to  of  these  amounts  should  be  used 
in  practice.  For  a  beam  supported  at  one  end  and  loaded  at 
the  other,  m  —  1  ;  uniformly  loaded,  m  —  supported  at  both 
ends  and  loaded  with  a  single  weight  at  the  centre,  m  —  \  \  and 
uniformly  loaded,  m  —  These  are  common  and  usual  con¬ 
ditions.  If  we  know  b  and  h,  we  can  then  find  W ;  or  knowing 
W,  as  in  the  examples  above,  we  can  find  h  by  giving  a  value 
to  b ,  or  find  b  assuming  h.  This  will  be  sufficient  for  the  pur¬ 
pose  now  considered  in  this  volume.  In  the  case  of  a  joist  in  a 
floor,  each  joist  supports  the  weight  on  an  area  of  the  floor 
equal  to  the  length  of  the  joist  multiplied  by  the  distance  from 
centre  to  centre  of  the  joists.  The  same  is  true  for  the  flooring 
of  a  highway  bridge.  The  load  is  generally  taken  as  that  of  a 
closely  packed  crowd,  and  estimated  at  from  50  to  100  lbs.  per 
square  foot  of  area.  For  a  warehouse  floor  the  actual  weight 
of  grain,  salt,  or  other  material  that  can  be  used  must  be  de¬ 
termined,  and  the  allowance  per  square  foot  thereby  determined 
in  each.  It  will  be  observed  that  the  uniform  load  on  a  beam 


JOINTS  AND  FASTENINGS. 


203 


has  the  same  effect  at  the  centre  as  a  single  load  at  that  point  of 
half  of  the  amount ;  therefore,  making  W  —  f  of  the  total  uni¬ 


form  load,  the  formula  reduces  to  \  Wl  — 


1  fbJi 

6~~r 


or  W=- 

3 


1/M3 


l 


for  a  beam  fixed  at  one  end  and  uniformly  loaded.  The 
general  formula  reduces  for  the  four  usual  conditions  of  load¬ 
ing  to 


IV- 


W-. 


IV. 


W: 


1/M3 
6  l  ■ 

1  fbie 

3  1 

2  fbh 1 

3  l  ' 

4  f_W_ 
3  l 


fixed  at  one  end  and  loaded  at  the  other. 

“  "  “  “  “  “  uniformly.  .  . 

supported  at  both  ends  and  loaded  at  centre. 

“  “  “  “  “  “  uniformly. 


(1) 

(2) 

(3) 

(4) 


The  value  of  /is  taken  from  tables.  It  is  important  to  note, 
when  using  the  tables,  in  what  units  /,  b ,  and  h  are  expressed, 
as  l  is  sometimes  in  inches  and  at  others  in  feet,  and  the  value 
of  /"is  given  to  correspond.  In  this  volume  /,  b ,  and  h  are  ex¬ 
pressed  in  inches,  and  /varies  for  timber  from  250  to  1500  lbs., 
according  to  degree  of  safety  required,  and  in  general  it  will 
be  from  \  to  the  ultimate  resistance.  The  value  of  W  will 
be  then  the  safe  load. 

72.  Timber  may  be  subjected  to  several  kinds  of  strain. 
1st.  Crushing  or  compressive.  2d.  Tearing  or  tensile.  3d. 
Transverse  or  bending ;  which  may  result  in  breaking  across  the 
grain.  4th.  Shearing ;  a  cutting  or  splitting  along  ^r  across 
the  grain.  5th.  Twisting  strain  or  torsion.  The  coefficient  or 
modulus  of  resistance  is  different  for  each  kind  of  strain,  and 
of  course  varies  with  the  kind  of  material.  Mr.  Rankine  gives 
for  oak  and  pine  timber,  for  either  crushing,  tearing,  or  trans¬ 
verse  strain,  10,000  lbs.  per  square  inch,  as  the  ultimate  resist¬ 
ance.  Green  timber  is  not  more  than  half  as  strong  as  seasoned 
timber.  The  resistance  to  crushing  along  the  grain  is  not 


204  a  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


more  than  half  to  two  thirds  its  tenacity  or  resistance  to  tear¬ 
ing.  The  resistance  to  shearing  along  the  grain,  600  lbs.  per 
square  inch  for  pine  and  2300  lbs.  per  square  inch  for  oak. 

Mr.  Trautwine  gives  the  following  table  of  the  ultimate 
strength  per  square  inch.  Green  timber  is  inserted  at  about 
two  thirds  that  seasoned.* 

To  resist  crushing  when  seasoned,  6,ooo  lbs.  per  sq.  in.  oak  or  pine. 


i  < 

it 

1 1 

‘  green, 

4,200 

i  t 

tt 

ti  ti 

t 1 

t  i 

tearing 

‘  seasoned, 

10,000 

1 1 

it 

tt  tt 

1 1 

t  ( 

<< 

‘  green, 

6,666 

tt 

i  t 

tt  a 

t  ( 

i  t 

cross-breaking  when  seasoned, 

10,000 

ti 

1 1 

oak. 

it 

t  t 

i  < 

“  green, 

6,666 

it 

1 1 

“ 

4  t 

tt 

t  ( 

“  seasoned, 

8,100 

ft 

tt 

white  pine. 

it 

ti 

it 

“  green. 

5-400 

tt 

tt 

i  t 

tt 

i  l 

•  t 

“  seasoned, 

9,900 

it 

*  t 

yellow  pine 

<  ( 

1 1 

“  green, 

6,600 

i  t 

1 1 

i  t 

it 

tt 

shearing, 

750 

tt 

1 1 

oak. 

a 

t t 

500 

tt 

ft 

pine. 

These  being  ultimate  or  breaking  strains,  the  safe  strain 
should  not  exceed  one  fifth  of  these  values,  and  for  heavy  rolling 
loads  not  more  than  from  one  eighth  to  one  tenth  of  these 
values. 

73.  The  strength  to  resist  tearing  is  independent  of  the 
length  of  the  member,  if  the  joints  and  connections  are  properly 
made  and  proportioned.  But  to  resist  crushing  the  strength 
decreases  rapidly  with  the  ratio  of  the  length  to  the  least 
dimension  of  the  member ;  and  when  its  length  is  from  20  to 
25  times  its  least  dimension,  its  resistance  to  crushing  is  reduced 
to  2000  or  2500  lbs.  per  square  inch.  But  in  this  case  a  special 

5000 

formula  should  be  used,  such  as  w  — - -p.  /=  length 

1  '  250 d* 

of  column  in  inches,  d  the  least  side  of  the  column  in  inches, 
and  w  the  ultimate  crushing  resistance  per  square  inch.  We 


*  If  /  in  formulae  1,  2,  3,  4  on  preceding  page,  is  in  feet,  the  ultimate  resist¬ 
ance  to  cross-breaking  in  this  table  must  be  divided  by  18  in  order  to  obtain  the 
value  of  f,  which  must  be  further  divided  by  factor  of  safety  to  obtain  safe  load. 


JOINTS  AND  FASTENINGS. 


205 


may  therefore  conclude  that  for  perfect  safety  the  following 
values  should  not  be  exceeded,  in  pounds  per  square  inch : 

To  resist  crushing  for  a  steady  load,  1,000  lbs.;  for  a  rolling  load  500  lbs. 
“  “  tearing  “  “  “  1,250  “  ;  “  “  “  700  “ 

“  “  cross-breaking  for  a  steady  load,  1,250  “  ;  “  “  “  700  “ 

“  “  shearing,  185  to  125  lbs. 


BILL  OF  MATERIAL. 


74.  Take,  for  example,  a  four-story  trestle  of  the  M  type, 
span  30  ft.  long  (which,  all  things  considered,  is  probably  the 
most  economical),  mortise  -  and  -  tenon  joint,  total  height  of 
bent  100  ft.  (See  Plate  IX,  Fig.  1.) 


Timber. 


Fourth  Story. 


Third  Story. 


Second  Story. 


First  Story. 


2  guard  rails . 

..  6"  X  6"  X  25' 

=  150 

25  cross-ties . 

.  6"  X  8''  X  10' 

=  1,000 

4  stringers . . . 

.  7"  X  15''  X  25' 

=  875 

2  bolsters . 

.10"  X  16"  X  6' 

==  160 

6  lateral  bracing . 

4^ 

X 

X 

0^ 

=  72 

I  cap . 

. .  12"  X  12"  X  10' 

=  120 

4  main  posts . 

.  .12"  x  12''  X  17-5' 

=  840 

2  X  braces . . 

. .  2"  x  10"  X  23' 

=  77 

4  longitudinal  braces. . . 

. .  8"  X  12"  x  33' 

=  1,056 

4  struts  under  stringers. 

..  8"  X  12"  X  20' 

=  640 

2  straining-pieces . 

. .  7"  X  12"  X  9' 

=  126 

I  cap . 

.12"  X  12"  x  17.5' 

=  210 

3  main  posts . 

.  .12"  X  12"  X  20.5' 

=  738 

2  “  braces . 

. .  8"  x  12”  x  20.5' 

-  328 

2  X  “  . 

. .  2"  X  12"  X  29.5' 

=  n8 

8  longitudinal  braces.  . .  , 

, .  6"  X  12"  X  31' 

=  1,488 

1  cap . . 

.  .12"  X  12"  X  23.2' 

=  279 

4  main  posts . 

.  .12"  X  12"  X  25.5' 

=  1,224 

2  “  braces . 

..  8"  X  12"  X  25.5' 

=  408 

2  X  “  . 

, .  2"  X  13"  X  33-°' 

=  143 

8  longitudinal  braces. . .  . 

■ .  6"  X  12"  X  3no' 

=  1,488 

I  cap . . . 

.  .12"  X  12"  X  32  0' 

=  384 

2  main  posts . 

.  12"  X  12"  X  34  7' 

=  833 

2  “  “  . 

,  .12"  X  14"  X  34-o' 

=  952 

2  “  braces . 

. .  8"  X  12"  X  34-7' 

=  555 

2  X  “  . 

, .  2"  X  12”  x  31.5' 

=  206 

longitudinal  braces . 

bottom  sill . . 

..12"  X  12"  X  43-8' 

=  426 

Total  timber . 

B.  M. 


206  a  practical  treatise  on  foundations. 


Iron. 


5  bolts  for  guard  rails. . 

• .  ¥  X  14" 

4  lbs. 

40  spikes  “  “  “  . . 
50  “  “  “  “  .. 

.IO" 

22 

it 

.IO" 

28 

(1 

8  bolts  for  stringers. . . . 

.  f"  X  16"  grip. 

l6 

it 

8  cast  packing  spools..  . . 

l6 

i  i 

4  bolts  for  bolsters . 

•  1"  X  25"  “ 

15 

i  i 

6  “•  “  straining  pieces  f  X  27  “ 

60 

it 

8  “  “  struts . 

•  f  X  24"  “ 

30 

it 

“  “  footplates... 

24  “  “  long,  braces. 

.  r  x  24”  “ 

90 

6  “  “  X  braces. . . . 

.  i”  X  16''  “ 

13 

“ 

4  “  “  “  “  - 

.  f"  X  14"  “ 

7 

8  drift  or  rag  bolts . 

.  1"  X  20"  “ 

24 

it 

4  lateral  brace  rods . 

. .  1"  X  6'  23"  grip. 

325 

66 

it 

(  < 

Total  iron. . 

i  i 

The  total  iron  should  also  include  the  weight  of  nuts  and 
washers.  Either  cast  or  wrought  washers  may  be  used.  Al¬ 
lowing  2  lbs.  for  head,  nut,  and  washers  for  each  bolt,  the 
aggregate  iron  would  be  555  lbs.  Allowing  $30  per  1000  for 
timber  framed  and  5  cents  per  lb.  for  iron,  the  above  bent 
would  cost  $475,  or  per  foot  $15.83. 

75-  This  calculation  has  been  made  on  the  M  form  of 
trestle  (see  Figs.  I  and  2,  Plate  IX),  which  shows  elevation, 
plan,  and  details.  It  is  probably  as  light  a  trestle  as  would  be 
good  practice  for  spans  25  to  30  feet  long  and  100  feet  high. 

76.  The  bill  of  material  is  given  purely  as  an  illustration. 
Any  other  form  of  trestle  can  be  similarly  calculated,  and 
comparison  as  to  cost  made.  It  is  better  in  approximate  esti¬ 
mates  to  overestimate  a  little  than  to  underestimate. 

77.  The  writer  does  not  give  the  extended  tables,  usually 
given  in  books,  of  the  strength  of  materials,  nor  the  vary¬ 
ing  results  of  different  experiments  on  the  same  material, 
It  has  been  his  sole  object  to  mention  those  timbers  in  com¬ 
mon  and  every-day  use,  that  are  likely  to  be  used  in  the  kind 
of  structures  considered,  and  only  to  give  those  values  of  the 
coefficients  of  strength  which  seem  to  be  universally  accepted 
as  fair  working  values  rn  actual  practice.  Extensive  tables 
are  given  by  Rankine,  Trautwine,  and  other  authors. 


TIMBER  PILES. 


207 


Article  XLII. 

TIMBER  PILES. 

78.  PILES  are  used  in  such  materials  as  are  not  able  to 
bear  the  weight  of  structures,  after  spreading  the  base  of  the 
structure  by  the  use  of  concrete  or  timber,  either  singly  or 
■combined,  or  where  the  cost  of  thus  preparing  the  foundation 
would  be  excessive,  and  also  where,  although  the  material  is 
firm  enough  to  bear  the  weight,  there  is  danger  of  it  being 
scoured  out  by  the  current,  thereby  undermining  and  endan¬ 
gering  the  structure,  and  often  without  considerations  of  the 
above  nature,  but  purely  on  account  of  convenience,  expedi¬ 
tion,  and  economy.  Piles  are  either  short  or  long  sticks  of  tim¬ 
ber,  generally  round,  sometimes  and  for  special  purposes  sawed 
square,  or  rectangular  as  in  sheet-piles.  They  are  driven 
into  the  ground  to  a  greater  or  less  depth,  depending  on 
the  purpose  for  which  they  are  used.  Oak,  pine,  cypress,  and 
elm,  are  the  principal  trees  used  for  piles.  Oak  has  the  advan¬ 
tage  of  being  hard  and  tough,  will  stand  more  hammering,  but 
cannot  be  obtained  as  large  or  as  straight  and  as  long  as  either 
pine  or  cypress ;  is  somewhat  more  expensive  in  certain 
localities,  mainly  on  account  of  the  cost  of  transportation. 
On  account  of  its  heaviness  it  is  apt  to  sink  in  water, 
and  large  rafts  are  liable  to  sink  unless  buoyed  up  by  some 
lighter  logs  intermixed  with  them,  such  as  poplar.  In  some 
localities  oak  is  more  abundant  than  pine,  and  is  consequently 
largely  used.  Pine  can  be  obtained  in  long,  large,  straight 
logs,  in  any  lengths  up  to  90  or  100  feet,  and  in  diameters  at 
the  butt  end  from  12  to  18  inches  or  more,  and  from  10  to  12 
inches  at  the  small  end.  The  yellow  pine  of  the  South  is 
hard  and  tough ;  these  qualities  make  it  particularly  useful 
for  piles,  and  owing  to  its  great  abundance  in  the  South  and 
the  fact  that  it  can  be  floated  in  large  rafts  on  the  many 
bayous  and  rivers  that  flow  through  the  forests,  it  is  com¬ 
paratively  cheap.  The  same  may  be  said  of  cypress,  but  this 


200  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 

splits  more  easily  and  does  not  stand  the  hammer  so  well. 
Elm  is  considered  good  for  the  purpose  also,  and  can  be  found 
in  great  abundance  in  some  localities,  but  does  not  seem  to 
be  used  to  any  great  extent  when  either  of  the  above  materials 
can  be  found. 

79.  Piles  are  prepared  for  driving  by  cutting  or  sawing  the 
large  end  square,  bringing  the  small  end  to  a  blunt  point  with 
an  axe,  the  length  of  bevel  being  from  l|-  to  2  ft.  long;  and, 
finally,  by  stripping  it  of  its  bark.  This  should  never  be  neg¬ 
lected,  certainly  for  that  part  below  the  ground.  In  soft  and 
silty  material  there  is  no  necessity  of  pointing  the  pile  at  all, 
and  in  fact  it  can  be  driven  in  better  line  when  left  blunt.  A 
pointed  pile  on  striking  a  root  or  any  obstruction  of  the  kind 
will  inevitably  glance  off,  and  no  available  power  can  prevent 
it ;  the  blunt  pile,  on  the  contrary,  will  cut  or  break  the  obstruc¬ 
tion  ;  ample  experience  fully  justifies  this  view.  The  large 
end  of  the  pile  is  chamfered  for  a  few  inches  from  the  end,  so 
that  a  wrought-iron  band  from  10  to  14  ins.  internal  diameter 
will  just  fit,  and  will  clasp  the  pile  uniformly  and  tightly  with 
one  or  two  light  blows  of  the  hammer.  Sometimes  a  ring 
from  1  to  l|-  ins.  less  diameter  than  the  pile  is  simply  placed  on 
the  top  of  the  pile  and  driven  into  it  by  light  blows.  This, 
however,  is  apt  to  split  long  layers  from  the  pile,  and  in  such 
cases  the  band  is  not  put  on  until  the  pile  is  more  or  less 
battered,  and  then  often  very  carelessly,  and  not  concentric 
with  the  end  of  the  pile.  The  first  method  of  fitting  the  ring 
to  the  pile  seems  to  be  the  best.  If  the  end  of  the  pile  is  not 
cut  square  and  true,  the  blow  will  be  received  on  one  edge  ; 
this  tends  to  split  the  pile,  to  drive  it  out  of  line,  and  break 
the  ring.  The  band  should  be  made  of  the  best  wrought-iron, 
with  metal  thickness  of  at  least  1  in.  and  3  in.  wide,  carefully 
and  thoroughly  welded  ;  with  every  precaution  the  rings  will 
very  often  break.  It  is  difficult  to  make  foremen  put  the 
ring  on  until  the  pile  begins  to  show  signs  of  splitting;  it  is 
then  too  late  to  be  of  much  advantage.  They  should  be  re¬ 
quired  to  put  them  on  in  the  beginning,  and  if  one  breaks, 
require  the  broomed  or  battered  portion  to  be  cut  off  and  a 


TIMBER  PILES. 


209 


new  ring  put  on  at  once.  It  may  be  easy  to  prevent  an  initial 
split,  but  difficult  to  prevent  it  extending  when  once  begun. 
Unless  bar  iron  is  convenient  and  can  be  obtained  readily,  a 
large  number  of  bands  of  different  diameters  should  be  pro¬ 
vided  in  advance,  as  rings  after  heavy  and  repeated  blows 
will  not  stand  many  weldings. 

80.  In  driving  piles  into  hard  and  compact  materials,  such 
as  stiff  clay,  sand,  and  gravel,  the  point  of  the  pile  is  often 
shod  with  iron.  Unless  this  is  properly  done,  no  great  benefit 
will  result,  and  as  commonly  done  it  is  of  little  use.  The  pile 
is  generally  brought  to  a  sharp  point ;  three  or  four  straps  of 
iron  are  welded  together  with  a  sharp  point,  both  inside  and 
out ;  the  end  of  the  pile  is  then  inserted,  only  touching  the 
straps  near  the  upper  ends:  bolts  are  then  passed  through 
the  straps  and  piles;  often  only  short  spikes  are  used.  Conse¬ 
quently,  the  bolts  split  or  cut  through  the  timber,  until  by  the 
force  of  the  blows  the  pile  is  made  to  fit  the  shoe,  or  the  straps 
spread  ;  thus  more  harm  is  done  than  good.  The  only  proper 
way  is  to  have  a  blunt  end  to  the  pile  from  4  to  6  ins.  in  diam¬ 
eter.  The  shoe  should  have  a  solid  conical  point,  the  base 
being  of  the  same  diameter  as  the  end  of  the  pile,  and  should 
fit  it  full  and  true  ;  the  straps  then  extending  upon  the  sides  of 
the  piles  and  bolted  to  them,  the  straps  and  bolts  mainly  hold¬ 
ing  the  shoe  in  place,  the  end  of  the  pile  receiving  the  effect  of 
the  blow.  Such  a  shoe  will  to  a  great  extent  prevent  the  end  of 
the  pile  from  brooming.  In  such  piles  as  the  writer  has  seen  that 
have  been  pulled  up,  he  does  not  recall  any  case  in  which  the 
lower  end  of  the  pile  has  split,  after  hard  driving,  even  without 
shoes  ;  but  the  end  of  the  pile  would  be  broomed  up  to  a 
length  of  6  ins.  It  would  be  interesting  and  instructive  if  the 
ends  of  many  piles  that  have  driven  could  be  examined.  It  is 
rare  that  piles  are  ever  pulled  after  being  once  driven  ;  it  is  far 
easier  to  cut  them  off  or  blow  them  off  below  the  bed  of  the 
river,  by  boring  holes  and  inserting  dynamite  cartridges.  We 
therefore  know  but  little  of  the  condition  of  the  points  of  the 
piles,  whether  driven  with  or  without  shoes. 

81.  We  do  know,  however,  a  great  deal  about  the  effect  cf 


210  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 

heavy  blows  on  the  upper  and  exposed  end  of  piles,  and  should 
be  able  to  learn  some  important  lessons  in  driving  piles  from 
these  effects.  There  is  always  great  danger  of  piles  splitting 
unless  well  banded  at  their  tops :  and  even  this  does  not  always 
prevent  it,  as  piles  will  often  show  a  split  and  a  consequent  buck¬ 
ling  below  the  band  and  extending  to  a  greater  or  lesser  dis¬ 
tance  downward;  but  unless  a  senseless  and  useless  hammering 
on  a  pile  is  required,  a  good  band  well  fitted  to  the  end  of  the 
pile,  and  the  end  cut  square,  will  prevent  any  serious  splitting. 
The  brooming  up  of  the  top  of  the  pile  will  take  place  as  a  rule 
whether  a  band  is  used  or  not ;  this  causes  the  head  to  swell 
and  bulge  and  the  bands  themselves  to  tear  apart,  and  oc¬ 
casionally  the  fibres  of  the  pile  are  completely  crushed  below 
the  band.  Even  when  piles  are  badly  broomed,  they  do  not 
necessarily  show  any  decided  or  dangerous  splits.  The  head  of 
a  pile  may  broom  to  a  considerable  extent  without  any  serious 
injury  to  the  body  of  the  pile  a  foot  or  two  below,  and  when 
cut  off  the  end  of  the  pile  should  show  a  hard,  firm,  uniform 
surface.  This  will  only  exist  where  a  band  has  been  used.  If 
any  great  brooming  results  where  a  band  is  not  used,  splits  to 
a  greater  or  less  extent  will  inevitably  exist.  The  writer  has 
personally  superintended  8  or  io  miles  of  pile-trestling  and 
numbers  of  pile  foundations  for  piers  and  abutments  in  all 
kinds  of  material,  and  has  more  than  casually  observed  as 
many  more,  and  now  recalls  but  few  instances  in  which  piles 
have  split  to  any  extent  when  properly  banded  and  banded  at 
the  proper  time,  unless  hit  many  heavy  blows  after  evident  re¬ 
fusal  to  penetrate  farther,  under  a  useless  law  based  upon  an 
equally  useless  formula.  As,  for  instance,  that  a  pile  shall  not 
penetrate  more  than  ^  to  ^  of  an  inch  by  30  blows  of  a  hammer 
weighing  2000  lbs.  falling  from  15  to  25  ft.,  at  each  blow,  and 
this  without  apparently  any  regard  to  the  depth  already  in  the 
soil  or  the  rapidity  of  the  blows.  The  above  is  a  liberal  rep¬ 
resentation  of  a  fact,  and  piles  are  hit  often  from  50  to  100 
blows  to  comply  with  such  requirements,  every  blow  brooming 
and  crushing  the  head  and  point  of  the  pile,  and  splitting  and 
crushing  the  intermediate  portions  to  an  unknown  and  danger- 


TIMBER  PILES. 


21  I 


ous  extent ;  the  piles  often  crush  between  the  head  and  the 
ground,  or  under  water  or  under  ground,  not  unfrequently  break¬ 
ing  short  off.  The  writer  ventures  to  assert  that  in  all  such 
cases  the  pile  does  not  move  at  all ;  the  apparent  penetration 
is  simply  due  to  crippling  the  fibres  at  some  point,  generally 
the  head  and  foot  of  the  pile,  but  often  at  intermediate  points, 
the  pile  supposed  to  be  moving  -gL-  of  an  inch  at  a  blow.  How 
this  infinitesimal  distance  can  be  determined  or  measured  in 
pile-driving  seems  hard  to  be  understood.  Piles  can  easily  be 
seen  to  bend  perceptibly  under  heavy  blows,  and  it  would  re¬ 
quire  perfect  elasticity  to  recover  their  exact  positions  within  -fa 
of  an  inch,  to  say  nothing  of  the  shortening  by  brooming  or 
crippling  of  the  fibres.  A  pile  may  go  from  £  to  ^  an  in.  appar¬ 
ently  for  each  blow  in  30,  and  never  actually  penetrate  the 
part  of  an  inch. 

82.  The  brooming  of  the  head  of  the  pile  has  the  effect  of 
materially  reducing  the  force  of  the  blow.  A  pile  may  ap¬ 
parently  have  ceased  to  move  under  repeated  blows  of  the 
hammer,  but  if  the  broomed  end  is  cut  or  sawed  off,  and 
then  struck  with  the  hammer,  it  may  readily  penetrate  several 
inches  at  a  blow  ;  but  it  is  hardly  ever  the  case  that  a  broomed 
end  pile  is  repeatedly  cut  off,  so  as  to  present  at  all  times  a 
hard,  firm  surface  to  receive  the  blow:  hence  formulae  would 
seldom  be  of  any  practical  value  in  determining  the  extent  to 
which  piles  should  be  driven.  There  are  many  formulae  pub¬ 
lished,  but  they  can  scarcely  be  considered  as  safe  guides  in 
settling  the  much-disputed  point  as  to  the  penetration  re¬ 
quired  to  bear  any  definite  load,  and  the  results  of  these  under 
apparently  the  same  conditions  are  so  different  that  they 
would  only  tend  to  confuse. 

83.  It  might  then  be  asked  if  formulae  are  of  no  value  and 
no  rules  as  to  the  penetration  allowable  in  the  last  10  to  30 
blows.  How  are  we  to  determine  when  to  stop  driving  a  pile. 
This  is  a  difficult  question  to  answer  directly,  for  many  reasons  : 

1.  It  depends  to  a  large  extent  upon  the  overlying  strata 
through  which  the  pile  has  been  driven.  A  long  pile  driven 
through  a  gritty  material  into  a  softer  underlying  strata  will 


212  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 

have  sufficient  frictional  resistance  to  bear  with  perfect  safety 
the  required  load  ;  this  resistance  being  reduced  to  minimum 
during  the  process  of  driving,  but  developed  to  its  full  extent 
after  an  interval  of  rest.  And  in  the  reverse  case,  that  of  a 
firm  soil  underlying  a  softer  stratum,  the  ability  of  a  pile  to 
bear  a  load  would  be  small,  notwithstanding  the  great  resist¬ 
ance  to  farther  penetration. 

2.  In  many  materials  a  very  great  resistance  will  be  de¬ 
veloped  when  the  pile  has  penetrated  only  a  few  feet — not  a 
sufficient  depth  to  give  steadiness  or  stability  to  the  structure 
or  to  be  safe  against  the  effects  of  a  scouring  action  of  the 
current.  The  writer  drove  about  2  miles  of  pile  trestle  in  an 
approach  to  a  bridge  across  the  Warrior  River,  Ala.,  in  a  com¬ 
pact  sand,  and  was  unable  with  a  1800-lbs.  hammer  to  drive 
white  oak  piles  more  than  from  5  to  6  feet  in  the  soil  without 
battering  the  piles  to  pieces.  Subsequently  this  was  filled  in 
with  earth.  A  constant  vibration  in  connection  with  the 
presence  of  water  would  doubtless  cause  more  or  less  settling 
of  such  piles  in  the  long  run. 

3.  Apparently  the  same  material  offers  a  very  different 
resistance  to  piles  driven  under  the  same  conditions.  In  some 
sands  piles  cannot  be  forced  over  a  depth  of  from  5  to  10  feet  : 
in  others  they  can  be  driven  from  20  to  30  feet,  and  again 
light  hammers  from  1500  to  2000  lbs.  with  a  high  fall  will  not 
be  as  effective  in  driving  piles  in  stiff  clay  or  sand  and  gravel, 
or  in  a  mixed  soil,  as  a  heavy  hammer  weighing  from  3000  to 
4000  lbs.  with  a  correspondingly  low  fall,  although  the  energy 
of  the  blow  is  the  same  in  both  cases;  the  blow  with  the 
high  fall  being  largely  taken  up  in  bending  and  brooming  the 
pile,  while  that  with  the  low  fall  seems  to  coax  along  the  pile, 
as  it  were.  Whether  this  results  from  the  fact  that  more  blows 
can  be  made  in  the  same  interval  of  time,  thereby  keeping  the 
pile  in  constant  motion,  rather  than  allowing  intervals  of  rests, 
or  from  some  other  cause,  the  fact  is  indisputable,  and  greater 
depths  can  be  reached  with  less  damage  to  the  pile. 

4.  In  driving  piles  in  certain  kinds  of  clay,  the  lateral  spring 
of  the  pile  makes  a  hole  perceptibly  larger  than  the  pile  itself, 


TIMBER  PILES. 


213 


thereby  allowing  surface  water  to  percolate  along  the  pile, 
often  as  deep  as  the  point  of  the  pile ;  and  whether  it  ever 
gripes  the  pile  is  an  unsettled  question.  This,  combined  with 
vibrations  from  a  rapidly  moving  train,  may  ultimately  cause 
settling. 

5.  No  rule  that  does  not  take  into  consideration  the  loss  of 
energy  resulting  from  broomed  ends,  the  varying  amounts  of 
frictional  resistance  of  different  materials  during  the  process  of 
driving,  and  the  depth  of  the  pile  in  the  soil,  but  is  based 
solely  on  the  weight  of  the  hammer,  the  height  of  the  fall,  and 
the  penetration  at  the  last  blow,  can  furnish  any  reliable  or 
even  approximate  idea  of  either  the  immediate  or  ultimate 
supporting  power  of  a  pile  or  any  number  of  piles. 

84.  We  must  therefore  rely  mainly  on  experience,  or  upon 
experiment,  in  each  particular  case ;  and  in  the  absence  of 
these,  it  is  merely  guess-work  and  taking  the  chances.  Exper¬ 
iment,  however,  is  in  the  reach  of  all,  will  cost  but  little  money, 
and  will  take  but  little  time ;  and  no  excuse  can  be  given  for 
not  making  satisfactory  tests  of  some  kind  in  the  absence  of 
precedents  in  similar  material  or  in  the  same  locality.  If  ex¬ 
perimenting  on  piles  in  trestle  work,  drive  a  single  bent  or 
two  bents  of  piles  at  the  proper  distance  apart.  Upon  these 
construct  a  platform,  and  place  weights  equal  to  or  twice  as 
great  as  the  greatest  load  that  can  possibly  come  upon  them. 
If  under  this  load  no  settlement  takes  place,  the  trestle  will  be 
safe,  and  piles  in  other  bents  driven  to  the  same  depth  and  to 
the  same  resistance  in  the  last  few  blows  can  be  relied  upon. 
If,  on  the  contrary,  settlement  does  take  place,  more  piles  or 
longer  piles  must  be  used.  Weights  can  gradually  be  added 
on  a  single  pile  until  it  Jpegins  to  settle,  and  from  this  the  num¬ 
ber  of  piles  can  be  estimated  to  carry  any  proposed  load,  allow¬ 
ing  a  factor  of  safety  from  2  to  4.  Clusters  of  piles  will  how¬ 
ever  bear  more  in  proportion  than  single  piles,  if  not  driven 
nearer  thar  2\  ft.  centres,  as  they  consolidate  and  compact  the 
soil  in  proportion  as  the  numbers  increase  in  a  given  area. 
Such  experiments  should  not  be  made  for  at  least  24  hours 
after  the  piles  are  driven,  so  as  to  allow  time  for  the  material 


214 


A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


to  compact  and  be  adjusted  around  the  pile.  The  following 
is  an  interesting  and  instructive  experiment  made  by  Maj. 
E.  T.  D.  Myers.  I  give  it  substantially  in  his  own  words,  as 
contained  in  a  letter  dated  Feb.  7,  1885  : 

“Bents  I2-J  ft.  centres,  6  piles  each;  length,  50  ft.;  Grade 
line,  15  ft.  above  low-water;  in  use  fourteen  years.  Piles 
driven  in  a  liquid  mud.  Two  bents  of  6  piles  each  were 
driven,  upon  which  a  platform  was  placed,  and  upon  this  a 
weight  of  75,000  lbs.  uniformly  distributed.  The  experiment 
was  made  19  hours  after  driving. 

OOOOOO  Bent  1 8th. 

OOOOOO  Bent  17th. 

123  456 

No  settlement  taking  place,  piles  Nos.  2  and  5  in  each  bent 
were  cut  out,  leaving  4  piles  in  each  bent.  Then  No.  3  of 
the  17th  and  No.  4  of  the  1 8th  bent  were  cut  out,  leaving  only 
3  piles  in  each  bent.  About  5000  lbs.  was  then  added  to  the 
load,  when  No.  6  of  18th  bent  yielded,  followed  by  No.  3  of 
the  same  bent,  and  sank  until  Nos.  4  and  5  were  again  brought 
to  bear.  It  required,  therefore,  about  13,000  lbs.  each  to  start 


the  piles. 

The  record  of  the  driving  was  as  follows 

BENT  17:  FALL  FROM  3  TO  10  FT. 

Pile  No.  1, 

1 1 

blows. 

Last  blow,  7  ft.  fall,  drove  it 

11 

nches 

“  “  2, 

13 

ii 

a  a  g  a  a  a 

9 

it 

a  a  2 

jy 

8 

it 

It  U  g  u  U  U 

18 

a 

“  “  4, 

8 

it 

U  U  g  it  u  u 

17 

ti 

a  a 

9 

a 

“  “  5  “  “  u 

6 

a 

«  “  6, 

7 

it 

“  “  7  u 

BENT  18.  # 

10J 

a 

Pile  No.  1, 

12 

blows. 

Last  blow,  5  ft.  fall,  drove  it 

io^-  inches 

“  “  2, 

8 

a 

U  ii  ^  U  it  it 

8 

a 

a  a  ~ 

jy 

8 

n 

tt  a  ^  u  u  it 

a 

“  “  4, 

9 

a 

n  a  ^  u  u 

4 

a 

it  il  r- 

Jy 

H 

a 

u  a  jq  a  it  a 

9 

a 

“  “  6, 

5 

a 

VO 

bo 

22 

a 

TIMBER  PILES. 


t 


215 


A  pile  40  ft.  long,  after  sinking  30  ft.  with  its  own  weight 
and  that  of  the  hammer  weighing  2000  lbs.,  was  struck  with 
a  blow  of  2-ft.  fall,  and  then  settled  6^  ins.  in  one  minute 
by  the  weight  of  the  hammer.  Four  weeks  after  this  a  blow 
with  a  fall  of  5  ft.  did  not  move  it.  A  blow  of  14-ft.  fall 
drove  it  4!  in.  Also  at  the  Gunpowder  River  piles  40  to  50  ft. 
long  were  driven,  until  they  did  not  sink  more  than  18  in. 
under  a  hammer  weighing  1800  lbs.  falling  20  ft.  Four  piles 
to  the  bent.  In  neither  case  was  a  hard  stratum  passed  through 
or  reached.”  This  is  but  the  common  experience  in  the  South¬ 
ern  swamps.  Even  with  very  light  falls,  the  penetration  at 
the  last  blow  is  from  4  ins.  to  2  ft.  High  falls  are  out  of  the 
question,  as  there  is  danger  of  losing  both  pile  and  hammer. 

In  all  cases  above  alluded  to,  these  trestles  have  carried  with¬ 
out  settling  the  heavy  trains  of  the  present  day.  The  above 
examples  show  the  great  load  that  piles  driven  to  a  depth  of 
30  to  35  ft.  in  the  softest  material  that  can  be  called  earth  will 
bear.  A  30-ft.  pile  that  is  30  ft.  in  the  soil  would  present  on 
an  average  about  90  sq.  ft.  of  surface  in  contact  with  the  soil, 
and  bearing  safely  13,000  lbs.;  the  frictional  resistance  would 
be  about  144  lbs.  to  the  sq.  ft.  of  surface.  The  probabilities 
are  that  they  would  carry  to  at  least  300  lbs.  The  frictional 
resistance  is  known  to  vary  from  300  to  800  lbs.  per  sq.  ft.,  de¬ 
pending  upon  the  nature  of  the  material  into  which  the  piles  are 
driven.  It  will  be  observed  in  the  above  table  that  pile  No.  6 
of  the  18th  bent  was  the  first  to  yield  under  the  weight  of 
13,000  lbs.  (This  pile  penetrated  under  a  9.8-ft.  fall  at  the  last 
blow  22  ins.)  The  effect  of  this  was  to  throw  a  large  portion  of 
the  13,000  lbs.  on  the  next  pile  in  the  same  bent,  which  of 
course  yielded.  The  greatest  load  that  could  come  upon  a 
span  of  I2-J- ft.  would  be  75,000  lbs.;  or,  in  a  four-pile  bent, 
would  be  18,750  lbs.,  and  in  a  six-pile  bent  12,500  lbs.  per  pile. 

85-  Some  8  miles  of  trestle,  constructed  under  the  writer’s 
direct  supervision  in  the  Southern  swamps,  the  bents  contain¬ 
ing  4  piles,  spans  12^  ft.,  depth  of  pile  in  the  soil  varying 
from  30  to  35  ft.,  the  penetration  varying  from  6  in.  to  2  ft. 
at  the  last  blow  of  a  2000-lb.  hammer  falling  only  a  few  feet.  ’ 


2l6 


A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


has  carried  for  twenty  years  the  heaviest  trains  without 
any  settling.  In  the  abutments  of  some  of  the  bridges  in 
these  swamps  the  piles  have  carried  with  perfect  safety  17,000 
lbs.  to  the  pile.  How  much  more  they  are  capable  of  carrying 
is  not  known.  In  one  of  these  abutments,  piles  only  30  ft.  in 
the  soil  could  not  be  moved  by  continued  hammering  with 
high  falls  a  few  days  after  driving.  The  experiment  was 
made  as  the  writer  was  not  satisfied  with  the  record  of  the 
original  driving,  and  desired  the  piles  to  be  driven  to  a  greater 
depth.  Finding  it  impracticable  to  move  the  piles  he  deter¬ 
mined  to  hammer  one  or  two  to  destruction  or  move  the  piles  ; 
destruction  was  the  result,  and  new  piles  were  driven  to  take 
their  place. 

86.  We  may,  therefore,  conclude  that  piles,  from  30  to  40, 
in  even  the  softest  alluvial  soils,  will  carry,  by  frictional  resist¬ 
ance  alone,  from  20,000  to  25,000  lbs.,  or  10  to  12^-  tons. 
There  are  examples  of  piles  driven  in  stiff  clay  to  the  depth  of 
20  ft.,  that  carry  from  70  to  80  tons  per  pile  ;  this  is  an  un¬ 
necessarily  heavy  load,  and  when  driven  from  2\  to  3  ft. 
centres  they  will  rarely  have  as  much  as  one-half  of  the  above 
loads  to  carry.  There  are  many  instances  in  which  piles  carry 
from  20  to  40  tons  under  the  above  conditions. 

87-  In  sand  and  gravel,  piles  will  carry  to  the  full  extent  of 
the  crushing  strength  of  the  timber,  provided  the  depth  in  the 
material  is  sufficiently  great  to  prevent  vibrations  from  reach¬ 
ing  the  point  of  the  pile  ;  other  considerations  will  require 
this  depth  to  be  at  least  10  ft.  or,  at  most,  20  ft.  Any  further 
hammering  on  piles  in  .such  materials  is  a  waste  of  time  and 
money,  and  injurious  to  the  pile  itself.  To  hit  such  a  pile  100 
to  1 50  blows  to  drive  it  an  inch,  as  has  been  done,  is  simply  folly. 

88.  Some  times  piles  drive  easily  and  regularly  to  a  certain 
depth,  and  then  refuse  to  penetrate  farther  ;  this  may  be  caused 
by  a  thin  stratum  of  some  hard  material,  such  as  cemented 
gravel  and  sand  or  a  compact  marl.  It  may  require  many  hard 
and  heavy  blows  to  drive  through  this,  thereby  injuring  the 
piles,  and  perhaps  getting  into  a  quicksand  or  other  soft  ma¬ 
terial,  when  the  pile  will  drive  easily  again.  If  the  depth  of 


TIMBER  PILES. 


217 


the  overlying  soil  penetrated  is  sufficient  to  give  lateral  sta¬ 
bility,  or  if  this  can  be  secured  by  artificial  means,  such  as 
throwing  in  broken  stone  or  gravel,  it  would  seem  unwise  to 
endeavor  to  penetrate  the  hard  stratum,  and  the  driving  should 
be  stopped  after  a  practical  refusal  to  go  with  2  or  3  blows. 
The  thickness  of  this  stratum  and  nature  of  the  underlying 
material  should  be  either  determined  by  boring  or  by  driving  a 
test  pile  to  destruction  if  necessary.  In  the  latter  case  the 
driving  of  the  remaining  piles  should  cease  as  soon  as  the  hard 
stratum  is  reached. 

89.  Sometimes  in  driving  piles  it  is  difficult  to  keep  the 
piles  down  after  the  impact  of  the  blow  is  over  :  the  piles,  begin¬ 
ning  to  rise,  lifting  the  hammer  with  it,  and  upon  removing 
the  hammer  the  piles  would  shoot  up  5  to  6  ft.  or  more.  This 
is,  no  doubt,  due  to  a  stratum  of  quicksand.  The  writer  has 
overcome  this  difficulty  by  driving  the  piles  with  the  butt,  or 
large  end,  downward.  This  is  the  only  case  in  which  piles  were 
driven  butt  downward,  in  the  writer’s  experience,  though  some 
authorities  recommend  it. 

90.  The  above  seems  to  cover  the  various  conditions  and 
kinds  of  material  met  with  in  driving  piles,  and,  as  can  be 
readily  seen,  no  general  or  rigid  rule  can  be  given,  either  as  to 
the  depth  to  which  a  pile  should  be  driven  into  any  kind  of  soil, 
or  as  to  the  penetration  in  the  last  blow,  or  last  few  blows. 
Experience,  and  experiment  alone  can  be  of  any  practical 
value.  But  enough  has  been  said  to  establish,  first,  that  when 
piles  are  driven  in  a  soft,  swampy  material,  a  penetration  of 
from  30  to  40  ft.  into  the  soil  will  give  ample  support  for 
ordinary  purposes,  regardless  of  the  weight  of  the  hammer,  the 
height  of  the  fall,  or  the  penetration  at  the  last  blow,  within 
limits  generally  existing  in  practice  ;  second,  in  clay,  sand,  and 
gravel  the  depth  required  is  only  that  necessary  to  give  stabil¬ 
ity  to  the  structure,  to  get  below  the  scour-line,  or  beyond  the 
reach  of  vibrations  caused  by  moving  loads,  in  general,  from  10 
to  20  feet ;  third,  that  a  continued  hammering  on  piles,  after 
practical  refusal  to  go,  is  absolutely  injurious. 

91.  The  following  formulae  are  given  for  the  benefit  of  those 


2J8 


A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


who  may  differ  with  the  opinions  expressed  above.  Rankine 
gives  the  following :  The  energy  of  the  blow  is  employed  as 


P7 

follows:  Wh  =  (employed  in  compressing  the  pile)  -f-  Px 

(employed  in  driving  it),  in  which  W  —  weight  of  the  ram  or 
hammer,  h  —  height  of  fall,  ;r  =  the  penetration  of  the  pile  at 
the  last  blow,  P  —  greatest  load  that  it  will  bear,  5  —  area  of 
cross-section  of  the  pile,  l  —  length  of  pile,  and  the  modulus  of 
elasticity  E  —  about  108,000  lbs.  per  square  feet.  Hence 


P  = 


aESWJi 

-7—+  ’ 


P 


2ESx 

l 


.  .  (I) 


In  any  particular  case  the  values  of  the  quantities  in  the 
second  member  are  all  known,  and  P  can  be  found.  Major 
Sander’s  formula  is  as  follows  : 


„  h  W 
P  ~  a  X  8  ’ 


(2) 


in  which  h  fall  in  inches,  W  —  weight  of  hammer  in  lbs., 
a  —  penetration  at  each  blow  towards  the  last,  P  =  safe  load 
in  pounds.  Trautwine’s  formulae, 


„  Vhw  X  0.0268 

Jp  1  1 

1  -f  a 

p  _  5°  Wh 

a  +  T  ‘  *  * 


(3) 

(4) 


and  for  the  safe  load  take  one  half  of  this  value.  Assume  the 
weight  of  the  hammer  at  2500  lbs.,  penetration  \\  inches  at  the 
last  blow,  or  towards  the  last.  Then  from  eq.  (2)  the  safe  load 


_  40  X  12  X  2500 
1.5X8 


100,000  lbs.  =  50  tons, 
% 


and  from  eq.  (3)  the  safe  load  equals 


P 

2 


3.42  X  2500  X  0.0268 
2.5  X  2 


=  49^  tons,  or  99,000  lbs., 


the  height  of  the  fall  being  taken  at  40  ft.  in  both  cases.  The 
calculation  in  Rankine’s  formula  is  long  and  tedious,  and  prob¬ 
ably  no  more  accurate.  Applied  to  piles  driven  with  a  hammer 


TIMBER  PILES. 


219 


1200  lbs.  and  fall  of  20  ft.  penetration  f  in.  Trautwine’s  formula 
gives,  as  a  safe  load,  24.9  tons,  and  Major  Sander’s  21.4  as  safe 
load  ;  the  actual  load  borne  by  the  piles  is  18  tons  to  each  pile  \ 
and  again,  piles  driven  only  16  ft.  into  alluvial  mud,  weight  of 
hammer  1500  lbs.,  fall  24  ft.,  penetration  2  in.,  actually  sup¬ 
porting  20  tons.  By  Trautwine’s  formula  safe  load  is  19.3  tons, 
and  by  Major  Sander’s  12.06  tons,  and  still  in  another  case  the 
calculated  safe  load  is  55  tons,  whereas  the  actual  load  is  70 
tons.  In  New  Orleans  the  piles  driven  from  25  to  40  ft.  carry 
safely  from  15  to  25  tons.  This  is  in  a  soft,  alluvial  soil. 


Article  XLIII. 

TIMBER  PILES— (CONTINUED). 

92.  There  has  been  suggested  recently  another  formula,  known 

fwh 

as  the  Engineering  News  formula,  as  follows :  P  —  — j., 

P  —  safe-bearing  resistance,  f  a  factor  varying  from  12  to  1, 
and  recommended  to  be  taken  -  2,  giving  a  factor  of  safety 
of  6,  w  —  weight  of  hammer  in  lbs.,  N  =  penetration  in  inches, 
the  average  during  the  last  few  blows,  and  C  taken  =  1,  a 
constant  to  provide  for  the  increased  resistance  to  moving  at 

2  wh 

the  moment  of  impact,  reducing  the  formula  to  P  —  for 

practical  use.  Since  writing  the  above  pages  on  pile-driving, 
this  formula  has  been  brought  into  great  prominence  by  rea¬ 
son  of  the  learned  and  able  discussions,  as  to  its  theoretical 
accuracy  and  practical  reliability  and  usefulness,  by  some  of 
our  leading  engineers.  The  conclusion  seems  to  be  reached 
that  it  is  certainly  as  reliable  as  any  of  its  predecessors,  and 
perhaps  comes  as  near  being  reliable  as  it  is  practicable, 
though  leaving  out  many  important  conditions  and  consid¬ 
erations,  which  must  materially  modify  the  relations  between 
the  energy  of  the  blow  and  the  penetration.  The  formula 
is  simple  and  easy  of  application  in  any  particular  case. 
The  writer,  however,  sees  no  reason  to  modify  the  already 
expressed  opinion  that  the  ultimate  bearing  resistance  of 


220  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 

piles  cannot  be  expressed  even  approximately  in  terms  of 
the  weight,  fall,  and  penetration  ;  and  even  if  approximately 
true  for  one  kind  of  material  and  one  set  of  conditions 
usually  attending  the  driving,  they  would  miss  by  very  far  the 
mark  when  applied  in  case  of  another  material  and  under 
other  conditions.  And  especially  as  his  experience  has  been 
confirmed  during  these  discussions  by  the  statements  of  many 
prominent  engineers  as  to  penetration  at  the  last  blow,  one 
engineer  stating  that  piles  about  40  ft.  long  sunk  with  their  own 
weight  and  that  of  the  hammer  the  full  length  of  the  pile,  and 
could  not  be  driven  at  all  after  a  period  of  rest ;  that  they  have 
carried  ever  since  the  heavy  trains  used  on  the  road. 

The  writer  has  sunk  piles  from  6  to  10  ft.  simply  by  work¬ 
ing  the  piles  backward  and  forward  ;  two  such  piles  to  the 
bent  carried  safely  a  construction  train,  loaded  with  rails  and 
ties,  for  many  months  without  any  evidence  of  settling. 

93.  After  a  period  of  rest  it  is  evident  that  piles  support 
their  loads  by  the  upward  pressure  at  the  point  of  the  pile  and 
by  the  frictional  resistance  on  the  surface  of  the  pile  in  contact 
with  the  soil.  The  relation  between  these  resistances  and  the 
weight  that  the  pile  can  carry  can  be  simply  expressed  as  fol¬ 
lows  :  w  =  p  -f-  fs,  in  which  w  =  the  safe-bearing  power,  p  —  the 
safe  resistance  to  settling  determined  by  the  bearing  power  of 
the  material,  fa.  factor  depending  upon  the  frictional  resist¬ 
ance  of  the  material  on  the  surface  of  the  pile,  .S'  =  number  of 
square  feet  of  surface  in  contact  with  the  soil.*  If  we  knew  p 
and  /"in  all  cases,  and  the  load  to  be  carried,  we  could  deter¬ 
mine  the  depth  of  one  or  a  group  of  piles  below  the  surface 
necessary  to  carry  the  load.  The  value  of  p  is  already  known 
approximately  for  ordinary  materials,  and  for  sand,  gravel,  and 
clay  is  universally  recognized  as  safe  at  5000  to  6000  pounds  per 
sq.  ft.,  and  for  silt  can  be  taken  at  zero.  The  value  of  f  can  be 
determined  with  the  same  degree  of  accuracy  as  is  now  used 

*  There  is  no  other  formula  applicable  to  the  bearing  power  of  piles  sunk  by 
the  water  jet,  or  worked  into  the  ground  by  a  to-and-fro  motion,  or  when  driven 
30  or  40  ft.  into  the  soil  by  three  or  four  blows  with  a  hammer  falling  three  or 
four  feet. 


TIMBER  PILES. 


221 


and  considered  safe  in  the  usual  coefficients  of  friction,  and  at 
a  comparatively  small  cost ;  and,  in  the  absence  of  more  reliable 
information,  it  could  be  taken  at  from  ioo  lbs.  in  the  softest 
semi-fluid  soils  to  200  lbs.  per  square  foot  in  compact  silt  and 
clay,  and  from  300  to  500  lbs.  in  mixed  earths  with  consider¬ 
able  grit,  and  from  400  to  600  lbs.  in  compact  sand,  and  sand 
and  gravel.  Assuming,  then,  that  we  were  driving  piles  for  a 
trestle,  4  piles,  to  the  bent,  bents  14  ft.  apart,  and  assuming  the 
equivalent  uniform  load  to  be  6000  lbs.  per  foot,  each  pile 
would  have  to  carry  21,000  lbs. 

In  the  silt  of  the  swamps,  with  p  =  o  and  f  —  150  lbs.  the 
w  — p 

formula  gives:  s  —  - ~  —  —  140  sq.  ft.  of  surface,  a  pile 

averaging  11  ins.  diameter  contains  2.8  sq.  ft.  per  foot  of 
length,  and  should  therefore  be  50  ft.  in  the  ground.  The 
writer  is  satisfied  that  a  bent  of  four  such  piles,  especially  if 
the  outside  piles  batter,  would  safely  carry  the  load.  If  con¬ 
sidered  risky,  put  in  a  centre  pile,  reducing  the  load  per  pile 
to  16,800  lbs. 

2.  When  driven  in  clay,  p  —  5000  lbs.,  and  f  —  150  lbs., 
each  of  the  four  piles  would  have  only  to  carry  fs  —  w  —  p  = 
21,000  —  5000  =  16,000  lbs.  by  frictional  resistance,  hence  s  — 
106  sq.  ft.,  or  depth  in  the  ground  =  38  ft.,  and  ii  /  =  200  lbs., 
s  =  80  sq.  ft.,  and  the  depth  in  the  ground  30  ft.  No  one 
would  question  that  ample  safety  is  secured  in  this  case,  and  in 
fact  15  to  20  ft.  in  the  ground  would  be  perfectly  safe. 

3.  In  compact  sand,  p  5000,/=  500,  s  —  $2  sq.  ft.  and 
depth  in  the  ground  =  12  ft.  nearly.  This  would  answer  in 
any  case,  unless  danger  from  scour  exists.  On  any  reasonable 
values  of  p  and  f,  the  above  formula,  I  think,  would  certainly 
be  equally  as  reliable  as  any  other,  and  certainly  comports 
better  with  the  actual  existing  conditions,  and  with  a  fair 
number  of  practical  tests  similar  to  those  already  described,  in 
varying  soils,  would  give  us  as  fair  a  standard  of  comparison, 
as  now  exists  in  the  case  of  retaining  walls,  timber,  and  iron 
columns  and  beams,  which  are  based  upon  experimentally 
determined  constants. 


222  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 

93b  We  can,  then,  conclude  that  the  bearing  power  of 
piles  will  vary  from  13,000  lbs.,  or  6^  tons,  to  140,000  lbs.,  or  70 
tons,  according  to  the  character  of  the  material  into  which  they 
are  driven  ;  and  this,  reduced  to  frictional  resistance  per  sq. 
ft.,  for  a  pile  driven  30  ft.  into  the  soil,  gives  from  140  lbs.  to 
1550  lbs.,  which  may  be  taken  as  the  extreme  limits,  and  from 
200  to  800  lbs.  may  be  taken  as  good  working  limits,  not  to 
exceed  the  smaller,  in  alluvial  and  soft  soils,  nor  the  greater, 
in  the  firmer  materials,  such  as  stiff  clay,  sand,  and  gravel  or 
mixed  materials. 

94.  The  usual  mode  of  driving  piles  is  by  means  of  the  pile- 
driver,  which  consists  essentially  of  2  horizontal  pieces  of  tim¬ 
ber,  10  X  12  ins.  or  12  X  12  ins.,  and  10  to  18  ft.  long,  placed 
about  3.5  ft.  apart,  connected  by  short  struts  and  tie  rods; 
near  one  end  of  these,  two  uprights,  6  or  8  ins.  by  10  ins.  and 
from  20  to  40  or  more  feet  long,  are  connected  by  cutting 
shoulders  in  the  ends,  so  as  to  fit  the  horizontal  pieces,  to  which 
they  are  also  fastened  by  bolts.  In  the  rear  of  the  verticals 
bent  iron  bars  are  placed,  the  ends  passing  through  the 
uprights  or  leads  ;  these  act  as  braces  for  the  leads,  and  also  to 
hold  the  wedges  necessary  to  keep  the  piles  in  place  and 
straight.  These  are  placed  at  intervals  of  from  6  to  8  ft.,  ver¬ 
tically;  on  top  of  the  leads  a  strong  cap  of  oak  or  some  hard 
wood  6  ins.  thick  and  12  to  15  ins.  broad,  is  placed  and  con¬ 
nected  by  mortise  and  tenon  and  iron  bolts.  A  ladder  runs 
from  the  other  end  of  the  horizontal  frame  nearly  to  the  top  of 
the  leads  to  which  it  is  bolted  ;  at  intervals  horizontal  pieces 
connect  the  ladder  and  the  leads,  upon  which  planks  are  placed 
for  platforms.  On  the  inside  of  the  leads  a  strip  of  hard 
wood  2 -g"  to  3  ins.  square  is  bolted  at  close  intervals,  and  on  the 
face  of  these-strap  iron  £  in.  thick  is  bolted,  the  heads  of  the 
bolts  countersunk.  A  cast-iron  hammer  of  the  required  weight, 
varying  from  IOOO  to  4000  lbs.,  is  cast  with  grooves  on  the 
sides,  so  as  to  be  held  in  place  by  the  strips,  and  at  the  same 
time  to  slide  freely  on  them.  A  rope  is  attached  to  the  upper 
end  of  the  casting,  and  passes  through  a  hole  bored  in  the  cap, 
and  over  a  pulley  fastened  on  top  of  the  cap,  thence  downward, 


TIMBER  PILES. 


223 


passing  through  a  snatch  block  at  the  bottom,  and  thence 
horizontally  to  a  drum  or  capstan ;  this  is  now  the  ordinary 
arrangement,  the  rope  being  permanently  fastened  to  the  ham¬ 
mer.  The  second  plan  is  to  attach  the  rope  to  a  heavy  double 
block  of  wood,  into  which  is  framed  a  pair  of  nippers,  with  the 
upper  ends  curved  outward,  and  the  lower  ends  with  pyramidal 
points  and  square  projecting  shoulders  on  the  inside  ;  these  are 
so  suspended  on  a  strong  bolt  that  the  lower  ends  remain  in 
contact,  and  are  only  opened  by  closing  the  curved  upper  ends. 
The  top  of  the  hammer  has  a  wedge-shaped  projection  with 
square  shoulders  a  few  inches  from  the  top  of  the  projection. 
The  block  is  also  framed  so  as  to  slide  down  the  strips  on  the 
leads  by  its  own  weight,  when,  in  falling  rapidly  on  the  ham¬ 
mers,  it  takes  hold  of  the  projection,  when  the  power  is 
applied  it  lifts  the  hammer  with  it,  when  it  comes  in  con¬ 
tact  with  bevelled  blocks  fastened  to  the  leads  near  the  top, 
the  curved  upper  ends  are  gradually  closed,  the  lower  ends 
open,  and  the  hammer  falls  ;  the  block  again  descends  rapidly 
and  clutches  the  hammer  as  before.  The  leads  are  braced 
laterally,  by  inclined  struts  resting  against  horizontal  pieces 
projecting  on  either  side,  and  the  rear  end  of  the  horizontal 
frame  must  be  weighted  or  held  down,  so  as  to  counterbalance 
the  weight  of  the  hammer.  The  first  method,  in  which  the 
rope  is  attached  directly  to  the  hammer,  has  many  advantages  ; 
more  blows  can  be  struck  per  minute,  the  height  of  fall  can  be 
more  easily  regulated  and  changed,  being  dropped  at  any  de¬ 
sired  distance  above  the  top  of  the  pile,  and  there  is  no  danger 
of  losing  the  hammer  if  the  pile  should  spring  out  of  the  leads 
or  be  driven  below  them.  (See  Plate  V,  Figs.  1,  2,  3,  and  4.) 

95.  Such  is  the  simple  pile-driver.  On  firm  ground  it  can 
be  moved  from  point  to  point  by  letting  it  rest  on  hardwood 
rollers  attached  to  the  under  side  of  the  horizontal  frame,  these 
rollers  being  turned  by  levers  inserted  in  holes  bored  into 
them.  On  softer  ground  a  platform  of  timbers  can  be  laid,  on 
which  the  driver  rests  and  is  moved.  This  is  a  slow  method, 
and  where  any  great  distance  is  to  be  passed  over  it  is  best  to 
fasten  the  driver  to  a  platform  made  of  strong  timbers,  upon 


234  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 

which  also  the  engine  and  boiler  can  be  placed,  the  whole 
then  resting  on  strong  timbers,  to  the  under  side  of  which  iron 
bars  or  rails  are  fastened.  This  is  elevated  on  a  level  with  the 
top  of  the  piles,  the  leads  project  beyond  the  platform  a  dis¬ 
tance  equal  to  the  distance  between  the  bents,  say  12^  feet; 
the  two  centre  piles  of  the  bent  can  then  be  driven.  The 
driver  is  moved  forward  a  few  inches,  the  frame  holding  the 
leads  can  be  turned  on  a  pivot,  and  the  two  outside  piles  driven 
in  a  proper  line  with  the  inside  ones.  These  piles  are  then  cut  off 
and  capped,  temporary  stringers  placed  in  position,  iron  cast¬ 
ings  with  grooved  rollers  fastened  to  the  stringers,  upon  which 
the  rails  run,  and  by  ropes  attached  to  the  bent  and  the  drum 
of  the  engine  the  driver  is  pulled  forward  into  its  new  position, 
and  another  bent  driven  as  before  described,  and  so  on.  In  a 
more  perfect  form  the  driver  is  attached  permanently  to  a 
platform  or  railway  car,  and  as  the  work  proceeds  stringers 
and  rails  are  temporarily  laid,  and  the  car  run  forward  on  these. 
For  the  repairs  of  completed  roads,  drivers  of  this  kind  are  used, 
the  leads  being  hinged  so  that  when  not  in  use  they  can  be 
lowered,  so  as  to  pass  through  bridges,  tunnels,  etc.  For  driv¬ 
ing  piles  in  water  the  driver  is  simply  fastened  to  a  barge,  and 
floated  to  its  position,  controlled,  held,  and  moved  for  short 
distances  by  means  of  anchors.  Piles  can  be  handled  more 
readily  and  more  economically  on  water  than  on  land,  but  it 
is  more  difficult  to  place  and  hold  the  driver  when  floating,  es¬ 
pecially  if  the  current  is  rapid,  or  in  high  winds.  The  cost  of 
driving  piles  should  not  exceed  8  or  ten  cents  per  lineal  foot  of 
pile.  The  cost  of  piles  vary  from  9  to  12  cents  per  lineal  foot. 

96.  The  power  used  in  pile-driving  is  either  man,  horse,  or 
steam  power.  The  first  is  not  often  used.  It  is  necessary  to 
have  a  light  hammer  and  a  low  fall.  A  number  of  men  take 
hold  of  a  rope,  lift  the  hammer  a  few  feet,  and  then  all  let  go 
at  the  same  time ;  it  is  a  slow  process,  and  not  calculated  to 
obtain  the  best  results. 

Horse  power  is  very  common  on  land,  and  can  be  used  on 
water;  the  rope  is  fastened  to  a  capstan,  which  can  be  readily 
made  by  any  carpenter  ;  a  long  lever  is  attached  to  a  centre 


TIMBER  PILES. 


225 


post,  to  which  a  horse  is  attached,  and  as  the  horse  moves  the 
rope  is  wound  around  the  capstan,  and  the  hammer  is  lifted  ; 
at  the  proper  moment  the  capstan  can  be  thrown  out  of  gear, 
and  the  hammer  falls.  Good  and  rapid  work  can  be  done  in 
this  manner. 

But  when  a  large  number  of  piles  are  to  be  driven,  steam 
power  is  mainly  used  ;  the  rope  is  attached  to  an  iron  spool 
connected  with  the  engine,  around  which  the  rope  is  wound  as 
the  power  is  applied,  and  by  throwing  it  out  of  gear  the  ham¬ 
mer  falls.  This  is  the  most  rapid  and  expeditious  method, 
and  admits  of  very  heavy  hammers  being  used. 

There  is  also  a  steam-hammer  pile-driver,  in  which  the  blow 
is  struck  by  a  hammer  attached  directly  to  the  piston  of  an 
engine.  In  this  very  powerful  and  rapid  blows  can  be  struck, 
and  doubtless  it  has  many  advantages  ;  but  it  is  not  in  common 
use,  and  in  fact  it  is  seldom  seen,  and  therefore  it  can  be  pre¬ 
sumed  to  be  less  economical  than  the  ordinary  drivers. 

97-  None  of  the  drivers  above  mentioned  can  be  used  to 
drive  piles  inclined  to  a  vertical  without  great  inconvenience 
and  delay,  but  it  is  often  desirable  to  drive  piles  on  a  batter,  this 
method  possesses  a  great  many  advantages  in  driving  piles  for 
trestles,  as  will  be  shown  in  another  paragraph.  A  pile-driver 
is  constructed  for  this  purpose  somewhat  differently  from  those 
described  above.  Instead  of  the  leads  being  fastened  to  the 
horizontal  frame,  they  are  supported  by  strong  heavy  bolts, 
attached  to  an  iron  frame,  which  is  fastened  to  the  horizontal 
frame,  the  leads  being  free  to  turn  about  the  iron  bolt  through 
an  arc  of  many  degrees,  by  which  means  the  leads  are  inclined 
to  the  vertical,  and  the  pile  can  be  driven  in  the  desired  direc¬ 
tion  with  the  same  rapidity  as  in  other  cases.  The  construc¬ 
tion  of  such  a  driver  is  as  simple  as  those  used  in  driving  only 
vertical  piles,  and  should  be  used  more  generally  than  it  is. 
The  drawings,  Figs.  1,  2,  3,  and  4,  Plate  V,  show  the  general 
construction  of  pile-drivers. 

98.  When  for  any  reason  it  is  necessary  to  sink  piles  to  a 
great  depth  in  a  firm  and  compact  material,  without  injury  to 
the  piles  by  many  heavy  blows,  it  can  be  done  by  the  use  of 


226  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 

the  water-jet  ;  a  pipe  can  be  attached  to  the  side  of  the  pile 
either  fitting  in  a  groove  cut  for  the  purpose  or  fastened  to  the 
outside ;  this  pipe  ending  in  a  nozzle  at  the  point  of  the  pile, 
the  upper  end  attached  to  a  force  pump  by  a  hose.  When 
the  water  is  forced  through  this  pipe  it  removes  or  loosens 
the  material  around  and  under  the  point  of  the  pile,  which 
sinks  by  its  own  weight,  or  a  weight  is  placed  on  top,  or  aided 
by  light  blows  from  a  hammer.  This  is  a  rapid  and  effective 
mode  of  sinking  piles,  and  has  been  used  to  a  large  extent  and 
with  satisfactory  results.  It  should  be  infinitely  preferred  to  the 
practice  of  long-continued  and  heavy  blows  to  drive  a  pile  a  few 
inches.  Piles  have  been  sunk  to  great  depths  by  this  process  ; 
it  is  an  application  of  the  same  principle  as  has  been  fully  ex¬ 
plained  in  sinking  the  Cushing  cylinders  and  in  making  borings 
for  foundations.  An  oblique  hole  is  sometimes  bored  into  the 
pile  near  the  bottom,  so  as  to  discharge  the  water  exactly  at 
the  point  of  the  pile  ;  but  this  does  not  seem  to  be  necessary,  or 
even  to  possess  any  material  advantage  ;  a  few  blows  should  be 
given  after  stopping  the  water-jet.  It  is  of  great  advantage  in 
sinking  iron  pipes  to  rock  for  the  purpose  of  submarine  blasting, 
or  for  columns  composed  of  iron  cylinders,  as  well  as  in  sinking 
screw-piles,  which  will  be  explained  under  that  heading.* 

Article  XLIV. 

USES  OF  PILES. 

99.  PURPOSES  for  which  piles  are  used  will  now  be  discussed. 
Piles  are  divided  into  long  and  short  piles,  or  piles  to  bear 
directly  and  entirely  the  load,  and  piles  the  main  object  of 
which  is  to  compact  a  soft  and  loose  material  so  as  to  increase 
the  bearing  power  of  the  soil.  Short  piles  or  those  used  for 
the  latter  purpose  are  from  8  to  15  ft.  long,  and  generally  from 
8  to  10  inches  in  diameter;  these  are  used  principally  to  sup- 

"x‘  Piles  were  recently  sunk  with  the  water-jet  to  the  depth  of  25  ft.  in  sand. 
It  is  stated  that  it  only  required  two  minutes  to  sink  each  pile.  Bowlders  under 
the  points  of  piles  were  carried  down  with  the  piles  by  sinking  a  pipe  to  the 
under  side  of  the  bowlders  and  using  two  water-jets  at  the  same  time.  These 
piles  could  not  be  moved  by  blows  from  a  heavy  hammer  only  a  few  minutes 
after  stopping  the  flow  of  water  from  the  pumps. 


TIMBER  PILES. 


227 


port  the  walls  of  houses  and  other  comparatively  light  struct¬ 
ures.  In  some  sections  of  the  country,  especially  in  the 
Southern  cities,  the  soil  is  of  a  soft  alluvial  material,  and  in  its 
natural  state  is  not  capable  of  bearing  heavy  loads.  In  such 
cases  trenches  are  dug,  as  in  firmer  material,  and  a  single  or 
double  row  of  short  piles  are  driven  close  together,  and  under 
towers  or  other  unusually  heavy  portions  of  the  structure  the 
area  thus  to  be  covered  is  filled  with  these  piles  ;  the  effect  of 
this  is  to  compress  and  compact  the  soil  between  the  piles  and 
to  a  certain  extent  around  and  on  the  outside,  thereby  increas¬ 
ing  its  bearing  power,  whatever  resistance  the  piles  may 
offer  to  further  settlement  may  be  added,  though  not  re¬ 
lied  upon.  These  piles  are  then  cut  off  close  to  the  bottom  of 
the  trench,  and  generally  a  plank  flooring  is  laid  resting  on  the 
soil  and  piles,  or  a  layer  of  sand  or  concrete  is  spread  over  the 
bottom  of  the  trench  to  the  depth  of  6  ins.  or  1  ft.,  and  the 
structure  whether  of  brick  or  stone  commenced  on  this.  There 
is  little  or  no  danger  of  such  structures  settling,  and  if  they  do 
the  chances  are  that  they  will  settle  uniformly  if  the  number 
•of  piles  are  properly  proportioned  to  the  weight  directly  above  ; 
but  if  the  same  number  of  piles  are  used  at  all  points  of  the 
structure,  although  considerably  great  weights  are  on  some 
walls  or  some  parts  of  a  wall,  unequal  settlement  may  take 
place,  causing  ugly  or  dangerous  cracks  in  the  structure. 

100.  A  modification  of  this  plan  is  to  drive  a  pile  into  the 
soil  and  then  withdraw  the  pile  and  fill  the  hole  thus  formed  with 
sand  ;  this  being  done  at  intervals  of  2  or  3  ft.  under  the  walls  of 
the  structure  as  above  described  and  all  of  the  holes  filled  with 
sand,  there  results  a  good  foundation.  The  columns  of  sand  are 
called  sand-piles,  owing  to  the  great  mobility  of  the  sand 
grains  ;  they  act  somewhat  as  in  a  fluid  pressure,  pressing  equal¬ 
ly  in  all  directions  at  any  given  depth,  and  therefore  afford  a 
better  support  than  the  wooden  piles,  and  have  the  further  ad¬ 
vantage  of  being  permanent.  The  wooden  piles,  unless  constant¬ 
ly  wet,  will  rot  sooner  or  later,  and  although  timber  constantly 
wet  does  not  rot,  yet  it  becomes  more  or  less  softened  and 
soppy  and  loses  some  of  its  original  strength. 


228 


A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


Short  piles  are  also  used  under  small  piers  and  abutments 
for  the  same  purpose  as  above  mentioned,  but  should  not  be 
used  where  very  heavy  weights  are  to  be  carried,  or  where  they 
can  be  reached  by  vibrations  caused  by  rapidly  moving  trains; 
in  such  cases  long  piles  should  be  used. 

IOI.  Long  piles  vary  in  length  from  15  to  100  ft.  and  in 
diameter  from  12  to  18  ins.  or  more;  and  although  their  re¬ 
sistance  to  settling  is  increased  by  compacting  the  soil,  the 
load  is  supported  generally,  though  not  always,  directly  by  the 
pile,  and  the  weight  does  not  come  upon  the  soil  between  them 
or  around  them  at  all.  Such  piles  are  always  used  when  great 
depths  are  to  be  reached  below  the  ground  or  water-surface,  or 
where  there  is  any  danger  of  the  material  around  them  scour¬ 
ing  out — as  under  very  large  and  heavy  piers  and  abutments 
in  rapid  currents,  or  in  very  soft  materials ;  also,  where  piers  or 
abutments  are  built  on  or  near  to  the  edge  of  steep  banks 
which  are  in  danger  of  caving  in,  under  the  action  of  rising  and 
falling  water,  although  the  material  itself  may  have  ample- 
supporting  power.  This  is  the  case  along  many  rivers,  notably 
the  Ohio,  whose  banks  cave  in  regularly  and  annually  at  very 
many  points,  unless  protected  by  stone-paving  or  vegetable 
growth,  willow  trees,  etc.  Should  such  banks  cave  under  the 
piers,  broken  stone  could  be  thrown  in  and  around  the  piles, 
thereby  securing  the  structure.  They  are  also  used  in  con¬ 
structing  wharves,  dykes,  etc.,  for  the  main  piles  in  coffer 
dams,  and  to  a  very  large  extent  in  building  railroads  across 
swamps,  bayous,  sloughs,  etc.  In  driving  piles  under  piers  and 
abutments,  the  piles  are  driven  in  rows  about  2%  feet  from 
centre  to  centre  of  piles  in  all  directions,  over  the  entire  area  to 
be  occupied  by  the  structure  and  one  row  on  the  outside,  mak¬ 
ing  the  area  about  i|-  to  2  feet  larger  all  around  than  the  bot¬ 
tom  of  the  structure  itself.  After  driving  the  piles  they  can 
be  cut  off  at  or  above  the  bottom  of  the  excavation,  and  upon 
these  caps  of  12  X  12  ins.  timber  are  placed  and  drift-bolted, 
another  layer  of  timber  placed  at  right  angles,  and  on  top  of 
this  another  layer  of  timber  or  plank,  all  bolted  or  spiked  to¬ 
gether,  and  the  masonry  started  on  this.  All  of  the  timber  should 


TIMBER  PILES. 


229 


be  under  the  lowest  low-water,  so  as  to  be  constantly  wet ;  in 
this  case  the  piles  bear  the  entire  load.  Sometimes  concrete, 
broken  stone,  or  gravel  is  placed  around  the  heads  of  the  piles 
and  around  and  between  the  timbers  of  the  platform,  and 
again  the  timber  may  be  entirely  omitted  and  concrete  placed 
around  and  over  the  piles  to  a  depth  of  at  least  2  feet.  In  the 
last  two  cases  a  part  of  the  load  is  borne  by  the  compacted 
soil  between  and  around  the  piles.  Opinions  differ  as  to  which 
is  the  better:  both  are  good  enough  ;  but  unless  the  timber  is 
wet  it  would  seem  better  to  leave  it  out,  though  some  authori¬ 
ties  say  that  timber  imbedded  in  cement  concrete  will  last  as 
long  as  when  constantly  immersed  in  water.  If  so,  the  first,  or 
rather  the  combined  timber  and  concrete,  would  be  preferred. 
Both  methods  are  in  common  use. 

102.  In  the  construction  of  wharves,  piles  are  driven  in  rows 
extending  well  out  into  the  water ;  the  distance  apart  of  the 
piles  and  the  depth  to  which  they  must  be  driven  depending 
entirely  on  the  load  which  they  have  to  bear.  These  are  cut 
off  a  few  feet  above  the  water  surface,  capped  with  square 
timber  10  X  12  ins.  or  12  X  12  ins.,  upon  which  joists  are  placed 
at  close  intervals,  and  on  these  a  plank  flooring  of  hard  wood 
3  ins.  thick.  Each  pile  in  such  cases  supports  an  area  whose 
sides  are  respectively  the  distance  between  the  rows  and  be¬ 
tween  the  piles  in  each  row — a  fair  average  being  5  ft.  each 
way,  or  an  area  of  25  sq.  ft.,  as  very  heavy  loads  are  often  con¬ 
centrated  over  small  areas.  The  proper  area  is,  however,  easily 
determined  when  the  greatest  load  per  square  foot  is  decided 
upon.  These  piles  will  soon  rot  above  the  water-line,  when  they 
must  be  cut  off,  and  framed  bents  of  timber  constructed  on  them. 
The  spaces  are,  however,  often  filled  up  with  earth,  gravel,  or 
shell,  and  only  a  timber  wall  or  bulkhead  must  be  maintained  at 
the  outer  end  to  hold  the  material  in  place.  This  consists  of  a 
timber  crib  resting  on  two  or  three  rows  of  piles,  tied  back 
into  the  embankment  by  long  timbers  notched  and  bolted  to 
the  crib  timbers.  In  front  fender  piles  are  driven  and  fastened 
to  the  crib  by  iron  straps  or  bolts,  and  projecting  above  the 
wharf  4  or  5  ft.  The  bulkhead  should  be  strong  and  heavy 


230  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


and  well  anchored  to  the  earth  as  the  weight  of  the  material 
behind  and  the  load  upon  it  exerts  a  great  force,  tending  to 
force  it  outward  ;  it  would  be  better  to  drive  the  piles  under  the 
crib,  slightly  inclined  backward,  so  that  the  resultant  pressure 
would  be  nearly  in  the  direction  of  their  length.  On  important 
water-fronts,  masonry  or  concrete  walls  or  bulkheads  are  con¬ 
structed  and  faced  with  timber,  against  which  boats  or  vessels 
can  rest.  Bulkheads  are  subjected  to  severe  blows  and  shocks 
from  vessels.  Such  constructions  are  used  along  the  foot  of 
embankments,  to  prevent  caving  and  sliding. 

103.  Dykes  or  jetties  are  formed  by  double  rows  of  piles, 
driven  at  right  or  acute  angles  with  the  direction  of  the  cur¬ 
rent,  sheeted  as  in  case  of  walls  of  coffer-dams,  and  filled  with 
gravel,  stone,  or  shells.  When  driven  at  a  slight  inclination  to 
the  current  they  serve  to  force  the  water  through  a  narrow 
channel,  and  the  increased  velocity  scours  the  material  of  the 
bed  of  the  river,  depositing  it  behind  the  jetties  or  in  the 
deeper  water  below  the  mouth  of  the  jetties.  When  at  a  large 
or  a  right  angle  to  the  current  they  serve  the  same  purposes. 
They  should  not  be  built  too  high  above  the  low-water  surface. 
These  subjects,  however,  more  properly  belong  to  river  and 
harbor  improvements,  and  are  merely  alluded  to  incidentally. 

104.  The  more  extensive  use  of  piles  for  foundations  is  in 
the  construction  of  trestles  across  swamps,  bayous,  etc.  There 
are  two  methods  practised;  one  is  to  drive  piles  in  rows  of  4 
or  6  each,  the  rows  being  from  10  to  25  ft.  apart,  the  position 
of  the  piles  in  each  row  or  bent  being  regulated  so  as  to  be  under 
the  posts  of  the  structure  above.  They  are  then  cut  off  a  little 
below  low-water  or  the  moisture  surface,  and  upon  these  framed 
trestles  of  any  height  and  of  any  of  the  forms  above  described 
are  constructed  and  fastened  to  the  piles  by  straps  or  bolts. 
This  has  the  advantage  of  placing  the  piles  where  they  will  do 
the  most  good,  and  of  utilizing  the  full  length  of  pile  to  support 
the  load  by  direct  bearing  or  by  the  frictional  resistance  of  the 
soil.  In  the  second  case  the  piles  are  allowed  to  project  above 
the  ground  or  water  and  then  cut  off ;  the  caps  resting  directly 
upon  the  tops  of  the  piles,  to  which  they  are  fastened  by  bolts 


TIMBER  PILES. 


t 


231 


or  straps.  This  method  is  hardly  applicable  when  the  height 
of  the  trestle  is  more  than  20  to  25  ft.  above  the  ground  or 
bed  of  the  streams,  owing  to  the  great  length  of  piles  required, 
and  the  further  fact  that  the  piles  are  driven  vertically,  and 
consequently  would  not  have  breadth  of  base  sufficient  to 
provide  lateral  stability,  and  it  is  therefore  limited  to  heights 
of  trestle  rarely  exceeding  15  ft.  above  the  ground.  This  is 
generally  as  great  a  height  as  will  be  required  above  the 
swamps,  and  commonly  not  more  than  5  to  10  ft.  When  no 
batter-posts  are  used,  the  common  construction  in  a  4-pile  bent 
is  to  drive  the  two  inside  piles  about  5  ft.  centres,  so  that  they 
may  be  directly  under  the  rails,  and  the  outside  piles  ft.  from 
these  on  either  side;  making  the  total  distance  from  outside  to 
outside  of  piles  from  12  to  13  ft.  This  requires  caps  about  14 
to  15  ft.  long,  generally  fastened  to  the  piles  by  drift-bolts. 
Upon  these  stringers,  cross-ties,  and  guard-rails  are  placed  sim¬ 
ilarly  in  every  respect  to  those  of  framed  trestles.  But  often 
additional  stringers  are  placed  over  the  outside  piles,  and  long 
cross-ties  13  to  14  ft.  long  are  used.  This  requires  a  consider¬ 
able  increase  in  the  quantity  of  timber  and  iron,  with  hardly  any 
compensating  advantages,  as  seen  by  the  following  comparative 
estimate.  For  a  single  span,  12^  ft.,  we  have 


1  cap  12  X  12  in.  X  14  ft . 

4  stringers  5  X  14  in.  12^  ft. . 
12  ties  6  X  8  in.  X  9  ft . 

2  guard-rails  6  X  8  X  12^  ft.  . 

And  in  the  second  case 


168  ft.  B.  M. 

350  “ 

432  “ 

100  “ 

- 1050.00  B.  M. 


1  cap  12  X  12  in.  X  14  ft .  168 

6  stringers  6  X  14  in.  I2|  ft .  525 

12  ties  6  X  8  X  14  ft .  672 

2  guard-rails  6  X  8  X  12^  ft .  100 

- 1465.00 

Or  an  excess  of  timber  for  each  12^ -ft.  length,  415.00  B.  M., 


equivalent  to  a  waste  of  175,300  ft.  B.  M.  per  mile  of  trestle, 
at  $26  per  1000  ft.  B.  M.=  $4558  per  mile.  Practically  but 
little  increase  of  strength  is  gained.  There  is  less  danger  of 


232  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 

a  train  wrecking  the  trestle  or  of  tumbling  off,  which  of  course 
is  an  important  consideration.  Whether  the  additional  safety- 
secured  is  worth  the  expenditure  of  the  above  sum  per  mile 
might  be  a  question  ;  but  this  can  hardly  be  justified  when 
framed  trestles  on  the  same  road  do  not  generally  provide  for  it, 
where  it  would  seem  that  every  precaution  for  safety  should  be 
provided,  as  the  trestles  are  much  higher,  and  consequently  the 
destruction  to  life  and  property  would  be  much  greater  should 
a  train  leave  the  track  and  be  thrown  from  the  trestle  ;  if  con¬ 
siderations  of  safety  control  in  one  case  they  should  control  in 
all.  It  is  evident  that,  in  a  4-pile  bent,  with  all  piles  vertical, 
the  two  piles  under  the  rails  have  nearly  if  not  all  of  the 
weight  to  carry,  the  outside  piles  only  acting  when  the  middle 
ones  settle.  For  this  reason  some  engineers  use  the  3-pile 
bents,  the  inner  piles  placed  half-way  between  the  rails  ;  the 
outer  piles  about  1  ^  ft.  from  the  rails  on  the  outside.  These  are 
capped  as  usual,  and  the  stringers  placed  between  the  piles  and 
only  one  half  the  distance  from  the  outside  pile  that  they  are 
from  the  middle  pile.  By  this  arrangement  each  outside  pile 
bears  two  thirds  of  the  weight  on  one  rail,  and  the  middle  pile 
one  third  of  the  weight  from  each  rail,  or  two  thirds  in  all.  All 
of  the  piles  will  bear  an  equal  portion  of  the  load.  But  the 
breadth  of  base  will  be  reduced  to  g\  to  10  ft.  from  outside  to 
outside  of  pile,  the  bent,  therefore,  will  be  wanting  in  lateral 
stability.  Moreover,  as  the  weight  is  supported  by  the  caps,  at 
a  point  between  the  piles  the  caps  will  have  a  bending  strain  to 
bear,  and  though  not  requiring  caps  of  greater  dimensions  than 
commonly  used  they  will  require  a  more  frequent  renewal  to  in¬ 
sure  perfect  safety.  In  this  trestle  the  only  saving  is  one  pile  for 
each  bent,  or  each  12^  ft.,  or  424  piles  to  the  mile  ;  assuming  the 
average  length  of  pile  at  40  ft.  we  save  16,960  lineal  ft.  of  piling 
at  30  cents  a  ft.  =  $5088.  For  low  trestles  there  will  be  suffi¬ 
cient  strength  and  stability.  The  caps  are  12  ft.  long,  so  there 
is  saved  also  about  10,000  ft.  B.  M.,  to  which  may  be  added  for 
saving  in  the  length  of  the  diagonal  or  X  bracing  some  5000 
ft.  B.  M.,  a  total  of  15,000  ft.  at  $26=^390,  or  a  total  saving 
of  $5478  per  mile.  Some  roads  take  advantage  of  this  saving 


TIMBER  PILES. 


233 


and  use  the  3-pile  bents,  but  the  4-pile  bents  are  preferred  and 
are  more  commonly  used. 

If  we  are  to  use  the  4-pile  bent  the  piles  should  be  driven 
so  that  they  may  all  do  full  service  not  only  in  bearing  the 
load,  but  in  adding  stiffness  and  strength  to  the  structure  ;  or,  in 
other  words,  all  trestles  should  be  built  in  such  a  manner  as  to 
make  the  most  advantageous  use  of  the  parts  composing  them. 
This  can  be  done  by  driving  the  outside  piles  on  a  batter  of  3 
ins.  per  vertical  foot,  the  same  as  that  used  for  the  batter  posts 
in  framed  trestles  (the  driver  for  such  piles  has  already  been 
alluded  to).  It  costs  no  more  than  the  4-pile  bent  with  all  ver¬ 
tical  piles;  is  as  easily  and  as  rapidly  built.  All  of  the  piles 
carry  a  portion  of  the  load  ;  the  outside  piles  give  stability  and 
stiffness  to  the  structure,  when  it  becomes  necessary  to  cut  the 
piles  off  below  water  surface  and  to  build  frame  trestles  on 
them  the  piles  are  in  the  proper  position  to  receive  the  weights 
to  be  carried — that  is,  in  the  prolongation  of  the  posts  of  the 
trestle,  both  vertical  and  inclined.  This  is  not  the  case  in  bents 
with  vertical  piles,  and  when  these  are  cut  off  the  batter-posts 
have  no  direct  support  under  them  unless  extra  piles  are  driven 
for  the  purpose,  ultimately  adding  848  piles  per  mile,  or  2  to 
the  bent.  It  would  be  equally  sensible  to  construct  the  framed 
trestle  bents  with  four  vertical  posts.  This  will  probably  be  real¬ 
ized  when  contractors  are  forced  to  burn  up  their  old,  rickety 
and  broken-down  drivers  that  have  been  in  use  for  a  quarter  of 
a  century.  In  a  3-pile  bent  with  outside  batter-piles  the  piles  at 
the  top  are  arranged  in  regard  to  the  rails,  as  in  the  vertical 
pile  bent.  But  whatever  may  be  the  height  of  the  trestle  bent 
its  base  is  spread  proportionally,  and  its  stability  is  secured.  A 
trestle  with  3-pile  bents  of  this  kind  has  more  bearing  power, 
more  stability,  and  costs  less  by  $5478  per  mile  than  a  4-pile 
bent  as  usually  driven  with  all  vertical  piles.  Whether  4-  or 
3-pile  bents  are  used  the  outside  batter  piles  should  be  used. 
These  four  types  of  trestle  bents  are  shown  in  Figs.  5,  6,  7, 
and  8,  Plate  VI,  respectively;  a  mere  glance  at  the  drawings 
establishes  the  truth  of  the  above  comparisons.  The  X  brace 
of  3-in.  plank  are  used  in  all  cases,  and  longitudinals  should  be 


234  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


used  when  the  trestles  are  over  io  or  12  ft.  in  height.  The 
decks,  composed  of  stringers,  ties,  and  guard-rails,  are  or  may¬ 
be  the  same  in  all  cases.  Figs.  9  and  10,  Plate  VI,  show  the 
elevation  and  plan  for  either  type  used. 

105.  Sometimes  it  is  necessary  or  desirable  to  use  pile 
foundations  either  of  wood  or  iron,  from  an  economical  point 
of  view,  when  the  bed  of  the  stream  is  of  a  rocky  material,  as 
on  coral  reefs  or  layers  of  marl  or  other  soft  stone,  into  which 
it  is  either  very  difficult  or  impossible  to  drive  piles  with  or 
without  shoes.  In  such  cases  holes  can  be  drilled  of  the 
proper  size,  a  fraction  less  in  diameter  than  the  piles  and  to 
the  required  depth,  and  piles  can  then  be  driven  firmly  into 
these.  The  depth  need  not  be  very  great, — from  1  to  3  ft., — as 
the  only  object  is  to  hold  the  bottom  of  the  pile  in  place,  or 
small  screw  disks  of  iron  can  be  fastened  to  the  bottom  of  the 
pile,  and  by  levers  or  other  suitable  machinery  the  piles  can  be 
screwed  into  the  material  to  a  sufficient  depth.  This  would, 
however,  be  rather  applicable  to  iron  shafts  or  columns  than  to 
wood.  Upon  these  suitable  platforms  are  constructed.  Such 
structures  are  frequently  constructed  for  various  purposes  on 
the  sea-coast.  It  must  be  remembered  that  timber  cannot  be 
used  in  such  situations  unless  creosoted,  as  it  would  soon  be 
destroyed  by  sea-worms. 

106.  Instead  of  drilling  holes  for  piles,  cribs  heavily  weighted 
with  broken  stone  could  be  sunk  on  the  bottom,  and  either 
timber  or  iron  trestles  could  be  built  on  these  and  well  bolted 
to  them.  In  this  manner  also  trestles  are  sometimes  built  across 
rapid  streams  with  rocky  beds.  A  timber  crib  is  framed,  with 
pointed  ends,  4  or  5  ft.  wide,  and  somewhat  longer  than  the 
bottom  sill  of  a  trestle  bent,  which  for  a  trestle  20  ft.  high 
would  be  about  24  or  25  ft. ;  this  is  sunk  in  place  until  one  end 
is  on  the  rock,  and  then,  while  suspended  in  a  horizontal  posi¬ 
tion,  broken  stone  is  forced  under  and  around  until  a  uniform 
and  solid  bearing  is  secured,  as  described  in  sinking  cribs  for 
piers  in  rocky  beds.  The  crib  is  then  filled  with  broken  stone, 
the  trestle  framed  on  the  crib,  and  secured  to  it  by  iron  straps. 
Plank  can  also  be  spiked  to  the  posts  of  the  trestle,  and  the 


COST  OF  TIMBER  TRESTLES. 


235 


inclosed  space  filled  with  broken  stone  to  add  to  its  stability. 
Such  trestles  will  stand  a  very  great  pressure  in  times  of  floods. 
The  bents  should  be  placed  in  a  plane  parallel  to  the  direction 
of  the  current  rather  than  perpendicular  to  the  line  of  road. 

Article  XLV. 

COST  OF  TIMBER  TRESTLES. 

107.  Timber  trestles  should  only  be  regarded  as  temporary 
expedients,  required  by  considerations  of  economy  or  rapidity 
of  construction,  or  sometimes  from  necessity;  and  it  is  generally 
expected  to  replace  them  by  earthen  embankments,  iron  via¬ 
ducts  or  trestles,  or  by  masonry  abutments,  piers,  and  bridge 
spans.  Such  considerations  should  not  be  allowed  too  much 
weight  in  designing  and  constructing  such  structures,  as  ex¬ 
perience  proves  that  they  often  remain  for  many  years,  repaired 
and  renewed  from  time  to  time ;  often  this  is  not  done  until 
some  disastrous  accident  or  wreck  occurs,  and  even  under  favor¬ 
able  circumstances  the  substitution  is  slow  and  gradual  and 
many  years  elapse  before  it  is  entirely  made. 

108.  When  framed  trestles  are  to  be  built  on  the  piles  of 
former  existing  pile  trestles,  a  record  of  the  original  pile-driving 
would  be  an  important  paper.  Unfortunately,  such  records  are 
not  usually  made,  and  if  made  not  preserved.  A  pile  trestle 
suitable  to  carry  the  comparatively  light  engines  and  loads  of 
20  years  ago  might  not  be  strong  enough  to  carry  those  of  the 
present  day,  unless  the  piles  were  originally  driven  to  the 
depths  and  resistances  previously  mentioned  as  safe  under  the 
present  heavy  loads.  The  writer  has  generally  kept  a  pile  re¬ 
corder  and  inspector  with  every  driver.  The  records  showed  the 
number  of  blows,  the  length  of  each  pile,  its  larger  and  smaller 
diameter,  and  its  penetration  at  each  blow  for  the  last  4  or  5 
blows,  and  the  depth  driven  in  the  soil,  the  number  of  feet  cut 
off  after  driving,  the  extent  of  brooming  or  splitting  if  any  ex¬ 
isted — in  other  words,  a  complete  history  of  each  pile.  All  that 
is  needed  is  an  honest  young  man,  who  will  obey  instructions. 
The  satisfaction  in  such  a  record  is  worth  all  it  costs ;  it  will 


236  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


make  known  the  weak  points  in  a  structure,  and  will  enable  the 
chief  engineer  to  have  a  full  knowledge  of  the  character  of  the 
work  done,  and  to  order  additional  piles  to  be  driven  in  certain 
places,  if  the  record  should  seem  to  require  it.  Daily  records 
of  the  progress  of  the  work  should  be  made  and  forwarded  at 
stated  intervals,  with  nature,  causes,  and  extent  of  delay.  This 
is  applicable  to  all  kinds  of  work,  and  at  all  times,  requires  but 
little  extra  labor  on  the  part  of  resident  engineers  and  inspect¬ 
ors;  makes  these  young  men  alert  and  observant,  and  keeps  the 
chief  engineer  fully  posted  on  matters  of  detail  and  importance, 
and  in  addition  such  records  are  of  great  value  to  the  company 
for  future  reference  and  use.  The  want  of  such  records  often 
costs  the  company  many  thousands  of  dollars,  while  the  cost  of 
having  them  will  amount  to  only  a  few  hundred  dollars.  A 
striking  case  of  this  kind  occurred  in  1886-87,  when  the  writer 
was  chief  engineer  of  the  Mobile  &  Birmingham  Railway.  A 
railroad  had  been  constructed  for  60  miles  out  of  Mobile  to 
the  Tombigbee  River  and  was  operated  for  several  years,  but 
owing  to  the  inability  of  the  company  to  renew  the  trestles  the 
aggregate  length  of  which  was  about  5  miles,  the  road  had  been 
abandoned  for  several  years.  This  road  was  constructed  in  1871- 
72.  Upon  assuming  charge  of  the  rebuilding  it  was  naturally 
concluded  to  cut  off  the  old  piles  below  low-water  line  and  to 
build  framed  trestles  upon  the  old  piles  ;  there  were  no  records 
made,  or  certainly  none  were  filed  among  the  papers  of  the 
company  that  were  preserved.  This  want  of  information  caused 
some  hesitation,  but,  believing  that  the  road  had  been  originally 
constructed  in  a  thorough  manner,  a  large  force  of  hands  was 
employed  at  several  different  points  to  cut  off  the  piles  ;  lengths 
of  trestles  were  measured,  bills  of  material  made  out,  and  par¬ 
tial  contracts  made.  In  the  mean  time,  all  of  the  old  piles  on 
several  trestles  had  been  cut  off ;  considerable  time  and  money 
had  been  spent  on  these,  when  unexpectedly  the  writer  was 
called  to  examine  the  trestles  at  one  or  two  points,  and  to  his 
surprise  he  found  that  in  many  cases  where  an  excavation  had 
been  made  to  get  below  the  line  of  constant  moisture  the  piles 
had  been  entirely  undermined  at  a  depth  not  exceeding  3  or  4 


COST  OF  TIMBER  TRESTLES. 


23  7 


feet.  Examinations  were  made  at  other  points  to  fi.  id  t,  le  points 
of  the  piles,  which  was  done  without  difficulty.  It  became  nec¬ 
essary  to  change  all  plans  and  contracts,  and  to  iepile  the  en¬ 
tire  distance.  Information  obtained  from  old  residents  pointed 
to  the  fact  that  originally  no  special  inspection  of  the  diiving 
had  been  made,  and  the  contractors  had  been  left  to  do  very 
much  as  they  pleased ;  that  if  a  pile  was  inconveniently  long  it 
would  be  cut  in  two  pieces  and  hit  a  few  blows,  and  that  the 
trestles  had  been  to  a  large  extent  built  in  this  way.  The 
business  on  this  60  miles  was  very  small ;  only  light  trains 
drawn  by  light  engines  had  been  run  over  it,  and  these  poor 
trestles  were  able  to  stand  the  small  loads  brought  on  them. 
After  discovering  this  condition  of  things  no  confidence  could 
be  placed  in  the  bearing  of  the  piles  in  any  of  the  trestles. 
To  change  orders  at  the  mills,  to  make  contracts  for  driving 
piles,  and  to  change  contractors,  cost  the  company  a  good  deal  of 
money  and  loss  of  time,  involved  the  company  in  suits ;  and  in 
some  instances  these  were  decided  against  the  company,  not¬ 
withstanding  the  fact  that  it  had  been  provided  in  the  contract 
that  the  right  to  change  plans  or  reduce  quantities  of  material 
was  specifically  reserved.  Had  a  regular  pile  inspector  been 
employed  such  work  would  not  have  been  allowed,  and  the 
chief  engineer  would  have  been  posted  in  regard  to  the  matter. 
Had  this  road  been  completed  in  the  beginning  and  heavy 
engines  and  trains  run  over  it,  many  of  the  trestles  would  inev¬ 
itably  have  given  way,  and  life  and  property  would  have  been 
destroyed. 

One  trestle  on  this  road,  about  1300  ft.  long,  was  built 
with  black  cypress  piles;  and  although  these  piles  were  rather 
small,  not  averaging  more  than  9  or  10  ins.  in  diameter,  had 
been  exposed  for  more  than  15  years,  many  of  them  were  found 
in  a  fair  state  of  preservation,  and  had  been  used  continually 
for  hauling  logs  to  a  saw-mill  in  the  neighborhood — a  small 
locomotive  running  over  it  constantly,  drawing  flat  cars  loaded 
with  lumber.  All  ot  the  pine  timber  originally  used  had  en¬ 
tirely  rotted  and  in  the  main  fallen  down,  though  in  places 
some  sound  timber  could  be  found. 

Some  engineers  do  not  cut  off  the  piles  and  build  framed 


238  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 

trestles,  but  repair  and  renew  the  trestles  by  driving  new  piles 
when  necessary. 

109.  A  comparative  estimate  will  now  be  given  of  the 
quantities  and  cost  of  framed  trestles  and  pile  trestles  of  the 
same  height.  We  will  assume  a  trestle,  the  total  height  of 
which  is  21  ft.  9  ins.,  the  framed  trestle  resting  on  mud  sills, 
bents  12^  ft.  from  centre  to  centre;  we  then  have,  fora  length 
of  12^  ft. : 


2  guard-rails  6  X  8ins.X  124  ft .  100  ft.  B.  M. 

12  ties  6X  8  “  X  9  “ . 432“  “ 

4  stringers  6  X  14  “X  124  “ . 350  “  “ 

1  cap  12  X  12  “  X  10  “ . 120  “  “ 

2  posts  12  X  12  “  X  19  “ . 456  “  “ 

2  batter-posts  12  X  12  “  X  igf  “  . . 471  “  “ 

1  sill  12  X  12  “  X  19!  “ . 234  “  “ 

6  mud-sills  12  X  12  “  X  5  “ . 360  “  “ 

2  diagonal  braces  3  X  12  “  X  26  “ . 156  “  “ 


2679 

Total  timber  in  a  length  of  12^  ft.  2679  ft.  B.  M.  at  $26.00= 
$69.65.  The  timber  in  the  pile  trestle  is  the  same  as  in  the 
frame  trestle  diminished  by  the  posts,  bottom  sill  and  mud-sills; 
or,  in  this  case,  1 1 58  ft.  B.  M.  at  $26.00  =  $30. 10,  and  increased 
by  4  piles  50  ft.  long  =  200  lineal  ft.  at  20  cts.  per  foot  =  $40.00, 
or  the  total  for  12^  ft.  =  $70.10,  a  difference  of  50  cts.  nearly  in 
favor  of  the  framed  trestle.  The  cost  of  the  iron  is  generally 
included  in  the  price  for  framing,  and  is  practically  the  same  in 
both  cases.  In  the  first  case  the  cost  per  foot  of  length  is  $5.57, 
and  in  the  second  $5.61.  The  above  prices  are  the  actual  con¬ 
tract  prices  paid  for  about  5  miles  of  trestle  in  1886.  For  tim¬ 
ber  framed  in  trestles  the  price  varies  from  $22.00  to  $30.00 
per  1000  ft.  B.  M.,  whereas  the  cost  of  driving  and  furnishing 
piles  varies  from  20  to  40  cts.  per  lineal  foot ;  and  although  in 
the  example  above  given  the  cost  of  the  two  trestles  are  prac¬ 
tically  the  same,  at  different  prices  between  the  limits  above 
given  there  might  be  a  wide  difference  in  the  relative  costs. 
In  general,  it  can  be  stated  that  the  cost  of  framing  timber  in 
trestles  including  the  iron  will  be  from  $8.00  to  $io.ooper  1000 
ft.  B.  M. ;  and  this  added  to  the  ascertained  cost  of  the  timber, 


COST  OF  TIMBER  TRESTLES. 


239 


delivered  and  piled  at  some  convenient  point  near  the  site  of  the 
structure,  will  give  a  close  approximation  to  the  cost  of  the 
completed  structure.  And  in  the  same  way  we  may  say  that 
10  cts.  added  to  the  cost  of  piles  per  foot  of  length  when  de¬ 
livered  at  the  site  of  the  structure  will  be  somewhere  near  the 
total  cost  per  foot  of  piles  when  driven  ;  but  this  is  more  liable 
to  variation  than  the  cost  of  framing,  as  the  handling  of  piles 
costs  more  in  some  cases  than  in  others :  if,  for  instance,  piles 
can  be  delivered  on  the  banks  of  a  stream  and  then  floated  to 
the  driver,  the  handling  will  cost  but  little  ;  if,  on  the  contrary, 
they  have  to  be  dragged  over  swampy  ground  for  any  distance, 
or  a  track  has  to  be  laid  and  the  piles  hauled  on  trucks,  the 
cost  will  be  greatiy  increased.  The  labor  of  cutting  off  piles 
to  receive  the  caps  is  generally  included  in  the  cost  of  framing; 
and  unless  the  same  party  does  the  driving  and  the  framing, 
the  party  driving  the  piles  will  add  something  for  the  cutting 
off,  and  the  party  doing  the  framing  will  not  make  any  deduc¬ 
tion  on  that  account.  And  in  either  case  the  amount  of  work 
to  be  done  in  any  one  place  materially  affects  the  cost  of  the 
work,  as  camps  have  to  be  established,  shanties  constructed, 
supplies  of  provisions  secured.  Every  move  of  camp,  transfer 
of  drivers,  machinery  and  tools  add  to  the  cost  of  the  work. 
The  cost  of  the  work,  however,  will  be  found  within  the  limits 
above  given — that  is,  for  framed  work  from  $22.00  to  $30.00 
per  1000  ft.  B.  M.,  and  from  20  to  40  cts.  per  lineal  foot  of 
piles  driven;  the  superior  limit  also  including  the  cost  of  cutting 
off  piles  under  water.  The  weight  of  round  iron  for  each  foot 
of  length  is  as  follows  : 


Diameters 

i  in. 

f  in. 

f  in. 

i  in. 

Weight  per  foot  . . . . . 

.  .0.66  lb. 

1 .00  lb. 

1 . 5  lbs. 

2.64  lbs. 

“  nuts  and  heads  . . .  . 

0.36  “ 

0.7  “ 

i-75  “ 

“  two-plate  washers.  . 

. .0.20  “ 

0.20  “ 

0.2  “ 

0.31  “ 

Total  weight . 

1 . 56  lbs. 

2.4  lbs. 

4.70  lbs. 

From  this  table  it  is  easy  to  calculate  the  amount  of  iron  and 
cost  per  bent  and  one  span  of  12^  ft.  in  length.  Some  engi¬ 
neers  use  a  little  heavier  bolt  than  others,  but  the  following 
will  be  ample  for  any  trestle.  Wrought  spikes  will  weigh  from 
to  |  lbs.  per  spike  10  ins.  long  and  \  in.  square  under  head. 


240  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


The  bill  of  iron  for  a  bent  of  trestle  and  one  span  of  I2f  ft. 
in  length  will  be  as  follows  : 


6  bolts  for  fastening  guard-rails  to  cross-tie,  $  in.  X  I  ft . 3.96  lbs. 

Nuts,  heads,  and  washers  for  the  same . 2.40  “ 

18  spikes  fastening  guard-rail  to  ties,  £  in.  X  10  in . 9.00  “ 

24  “  “  ties  to  stringer,  ^  in.  X  10  in . . . 12.00  “ 

2  bolts  for  fastening  stringer  to  cap,  f  in.  X  2'  4" .  3.60  “ 

Nuts,  heads  and  washers .  1.80  “ 

8  bolts  for  bolting  stringers  together,  f  in.  X  lift . 12.00  “ 

Nuts,  heads,  and  washers .  4.48  “ 

12  bolts  for  fastening  X-bracing  to  posts,  cap  and  sill,  i  X  1$  ft . .  11.88  “ 
Nuts,  heads,  and  washers .  4.80  “ 


Total  iron  for  each  12^  feet  of  trestle . 55-72  lbs. 

In  case  of  pile  trestle  add  4  drift  bolts,  1  in.  X  2  ft . 21.16  “ 

And  if  longitudinal  bracing  is  used,  4  bolts  i  in.  X  14  ft .  3.96  “ 

Or  total  iron . 80.84  lbs. 


If  straps  ^  in.  X  ii  in.  X  2  ft.  are  used  instead  of  drift  bolts, 

8  straps,  5  lbs.  each  =  40.00  lbs. 

16  bolts  i  in.  X  15  in.  =  13.12  “ 

Nuts  and  heads  =  3.20  “ 

Total . 56.32  lbs.  less  drift  bolts  21.16  lbs.  =r  35.16  lbs. 

Total . . . . 116.00  lbs. 

In  the  three  suppositions  above,  the  amount  of  iron  per  foot 
of  length  of  trestle  is  respectively  4.45  lbs.,  6.47  lbs.,  9.28  lbs., 
and  the  cost  at  5  cts.  per  pound  is  respectively  22^  cts.,  32^  cts., 
46|  cts.  These  differences  seem  small,  but  they  amount  to  a 
considerable  sum  in  a  mile  of  trestle.  The  second  costs 
$532.40  more  than  the  first,  and  the  third  costs  $1325.12  more 
than  the  first,  and  $792.72  more  than  the  second  for  each  mile. 
It  is  often  necessary  to  use  the  straps,  notwithstanding  the  in¬ 
creased  cost,  and  in  addition  when  a  trestle  is  to  be  renewed 
the  straps  can  be  used  over  again,  whereas  drift  bolts  can  rare¬ 
ly  be  used  a  second  time ;  and  for  this  reason  straps,  though 
more  costly  at  first,  will  prove  ultimately  to  be  the  most  eco¬ 
nomical,  and  in  addition  lessen  the  labor  and  cost  incident  to 
repairs  and  renewals.  Instead  of  using  screw  bolts  for  the 
longitudinal  and  diagonal  braces,  either  wrought  or  cut 
spikes  are  frequently  used  ;  ultimate  economy  will  result  by 
using  the  bolts,  besides  other  advantages.  This  subject  has 


COST  OF  TIMBER  TRESTLES. 


24I 


been  discussed  more  in  detail  than  it  apparently  deserves  ; 
but  the  importance  of  such  things  is  apparent  to  both  engineer 
and  contractor,  and  it  will  often  be  found  that  a  little  knowl¬ 
edge  on  these  seemingly  small  and  unimportant  subjects  will 
be  of  inestimable  value  to  the  engineer.  Bolts  are  simply  used 
as  a  rule  without  any  regard  to  the  actual  diameters  required — 
i-in.  bolts  used  where  a  f-in.  bolt  would  be  sufficient,  and  f-in. 
bolts  used  where  a  f-  or  £-in.  bolt  would  answer  every  purpose. 
A  glance  at  the  foregoing  table  shows  that  f-in.  bolt  with  head, 
nut,  and  washers  weighs  2.4  lbs.,  whereas  an  inch  bolt  weighs 
with  nut,  head,  and  washer  4.7  lbs.  for  a  bolt  1  ft.  long,  that  is, 
about  twice  as  much. 

1 10.  When  the  bents  are  placed  farther  apart,  or  when  the 
spans  are  longer,  the  number  of  bents  to  the  mile  are  less, 
thereby  saving  material ;  but  the  length  of  the  stringers  re¬ 
quired  causes  an  increase  in  their  dimensions  or  in  their  number 
or  in  the  timber  required  in  the  straining-beams  and  struts,  and 
some  increase  in  the  number  of  bolts.  With  any  given  height 
of  trestle  it  would  be  an  easy  matter  to  determine  the  economi¬ 
cal  length  of  span  to  use,  as  the  dimensions  of  the  timber  in 
the  bents  themselves  are  practically  the  same  in  spans  from  12^ 
ft.  to  25  ft.  in  length.  In  the  shorter  spans  there  is  always  a 
large  excess  of  timber  in  the  bents  above  that  actually  required 
to  carry  safely  the  loads,  as  the  four  posts  together  have  a  total 
cross-section  of  at  least  528  sq.  in.  area,  or  a  safe  resistance  to 
crushing  at  500  lbs.  per  sq.  in.  of  264,000  lbs.,  whereas  the 
total  load,  rolling  and  fixed,  would  not  exceed  90,000  lbs.,  and 
for  the  25-ft.  span  the  total  load  will  not  exceed  140,000  lbs.; 
therefore  up  to  this  limit,  economy  would  justify  the  long  spans 
rather  than  the  short  ones.  But  as  the  spans  increase  the  sup¬ 
ports  or  foundations  of  the  bents  must  be  stronger,  and  in  pile 
trestles  more  piles  to  the  bents  would  be  required,  unless  the 
material  into  which  they  are  driven  is  firm  and  compact.  En¬ 
gineers  have  apparently  settled  on  certain  lengths  of  spans 
without  considering  the  question  of  economy,  and  we  therefore 
find  the  standard  spans  for  a  single-story  trestle  either  12J  or 
14  ft.,  occasionally  1 5  ft.,  and  for  trestles  of  two  or  more  stories 
20-  and  25-ft.  spans,  the  dimensions  of  stringers  for  the  spans 


242  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 

seem  to  be  determined  in  the  same  arbitrary  way;  and  we 
find  for  the  spans  I2f,  14,  15  ft.  the  following  dimensions  re¬ 
spectively:  4  pieces  each  6  X  14  ins.,  7X15  ins.,  and  8  X  16 
ins.  Applying  the  formula  m  Wl  —  nfb/f  to  these  three  cases, 
W  being  the  equivalent  centre  load,  or  one  half  of  the  uniform 
load  on  the  clear  spans,  /=  1 1£,  13,  and  14  ft.  respectively,  and 
the  load  6000  lbs.  per  ft.  is  equal  to  8625,  9750,  and  10,500  lbs. 
respectively  ;  f  —  -jf,  b  —  6,  7  and  8  ins.,  and  /  =  138,  156  and  168 
ins.,  respectively.  We  find  h—  ins.,  18  ins.  when  f—  1000  lbs., 
and  14  ins.,  14-4-  ins.,  and  15  ins.  respectively,  when  f  —  1500  lbs. 
We  may  then  conclude  that  theoretically  the  assumed  dimen¬ 
sions  of  the  stringer  are  sufficient.  For  the  spans  20  and  25  ft. 
the  stringers  can  be  of  the  same  dimensions  as  given  above; 
the  dimensions  of  the  struts  or  rods  used  in  bracing  them  being 
determined  by  the  lengths  of  stringers  supported  directly  by 
them,  as  explained  in  paragraphs  33  and  70.  We  will  now  de¬ 
termine  by  formula  the  depth  of  stringer  required  when  six 
string-pieces  are  used  instead  of  four,  as  above  considered.  In 
this  the  total  load  (one  half  of  the  total  uniform  load)  divided 
by  six  will  give  the  equivalent  concentrated  single  load  at 
the  centre  as  illustrated  in  the  following  diagrams  for  the 
three  different  lengths  of  span.  W  then  in  the  formula  will 

34>_5^  39,000,  4^,000,  equa]  respectively  to  5750,  6500,  and 

7000  lbs.,  the  value  of  all  other  quantities  as  above,  from  which 
the  values  of  h  or  the  depth  of  stringer  in  inches  will  be  respec¬ 
tively  (for  y  =  IOOO)  14  ins.,  14I  ins  ,  15  ins.,  and  (for /=  1500 
lbs.)  li|-  ins.,  12  ins.,  13  ins.  Therefore  we  can  use  stringers 
under  each  rail  composed  of  two  pieces  each  6  ins.  X  14  his.,  7  ,ns- 
X  14I  ins.,  8  ins.  X  1 5  ins.,  or  three  pieces  6  ins.  X  1 1|  ins.,  7  ins 
X  12  ins.,  8  ins.  X  13  ins.  for  the  spans  respectively  of  I2|,  14, 
and  15  ft.,  centre  to  centre  of  bents  with  equivalent  strength  in 
each  case,  and  either  of  these  for  the  20  and  25  ft.  spans,  when 
properly  trussed  as  already  explained. 

ill.  Shall  we  use  then  the  standard  12^-ft.  spans  for  a  single- 
story  trestle  or  15-ft.  spans?  We  have  seen  in  paragraph  109 
that  the  cost  of  a  12-ft.  span  of  framed  trestle  is  $5.57  per  foot 
of  length,  and  that  the  amount  of  timber  was  2679  ft.  B.  M.  in 


COST  OF  TIMBER  TRESTLES. 


243 


a  span  of  12^  ft.  Deduct  from  this  the  guard¬ 
rails,  cross-ties,  and  stringers,  amounting  to  882  ft. 
B.  M.  there  remains  1797 — to  which  add 


4  stringers  8  in.  X  15  in.  X  15  ft.=.  .  600 

15  cross-ties  6  in.  X  8  in.  X  9  ft.=.  .  540 

2  guard-rails  6  in.  X  8  in.  X  15  ft. =.  .  120  1260 


t 


We. have  total  timber  in  a  span  of  15  ft.  in  ft.  B.M.  .  .  3057 


which  at  $26  per  1000  —  $79.48,  or  cost  per  foot 
of  length  $5.30,  a  saving  of  27  cts.  per  foot,  equiva¬ 
lent  to  a  saving  of  $1425.60  per  mile  of  trestle.  If 
six  string  pieces  8  ins.  X  13  ins.,  X  15  ft.  =  780  ft. 
B.  M.,  or  180  ft.  more  timber,  equal  to  $4.68  per 
span,  or  31  cts.  per  foot,  making  the  cost  in  that 
case  $5.61  per  foot,  which  shows  that  the  substitu¬ 
tion  of  8  X  13-in.  stringers  is  not  an  economical  use 
of  stringers,  as  might  have  been  expected  ;  but  it 
may  often  be  difficult  to  secure  8-in.  X  15-in.  string, 
ersof  clear  heart,  whereas  the  8-in.  X  13-in.  string¬ 
ers  could  be  secured.  But  this  is  a  little  more 
expensive  than  the  spans  of  I2-J  ft.  long  with  6-in. 
X  14-in.  stringers  ;  and  if  this  size  of  stringer  can 
be  obtained  it  would  be  preferred,  as  there  would 
be  less  pressure  on  the  supporting  material, whether 
mud-sills  or  piles.  The  above  considerations  and 
principles  have  a  very  much  more  important  appli¬ 
cation  when  deciding  upon  the  economical  rela¬ 
tions  of  piers  and  length  of  spans  in  long  bridges, 
as  in  such  cases  the  economical  length  of  span  is 
a  matter  of  very  great  importance.  But  in  this 
place  it  will  be  sufficient  to  say  that  as  a  general 
rule  in  low  structures  the  spans  should  be  short 
with  many  supports  or  piers,  and  in  high  structures 
the  spans  should  be  long  and  with  few  piers  or 
supports.  This  will  be  considered  more  fully  in  a 
subsequent  chapter. 

1 12.  It  may,  however,  be  stated  here  that  in 


Uniform  loaci=69000  lbs. i  j  Uniform  load =78000  lbs.  j  I_ Uniform  load=  84000  lbs. 


244 


A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


very  high  trestles  it  will  be  better  to  use  spans  from  40  to  50  ft. 
or  more,  and  to  construct  piers  or  double  bents  similar  to  the 
timber  piers  described  in  paragraph  28.  This  can  only  be  de¬ 
cided  by  a  careful  estimate  of  cost,  including  the  extra  precau¬ 
tions  required  to  secure  safe  foundations.  The  principles  in¬ 
volved  have  been  fully  discussed  in  the  preceding  paragraphs. 

1 13.  As  a  general  rule  contracts  provide  for  payments  on 
the  basis  of  so  much  per  1000  ft.  B.  M.  framed  trestles,  and  so^ 
much  per  lineal  foot  of  piles  driven,  and  sometimes  in  addition 
so  much  per  pound  for  iron  used.  Sometimes,  however,  the 
contract  is  so  much  per  lineal  foot  of  completed  trestle.  This 
last  method  possesses  many  advantages,  as  in  this  case  there 
can  be  no  dispute  as  to  the  final  settlement.  The  work  shows 
for  itself ;  either  party  can  measure  the  length.  In  other  cases 
questions  may  and  do  arise  every  month  ;  the  contractor  is  not 
satisfied  with  his  estimate,  complaints  are  made,  and  extra  bills 
presented.  It  is  difficult  to  provide  for  every  contingency  in 
contracts— whether  the  lengths  of  posts  mean  from  end  to  end 
of  tenons,  or  whether  the  tenons  are  to  be  excluded  ;  how  the 
cut-off  ends  of  piles  are  to  be  paid  for,  and  packing  blocks 
between  stringers ;  excavations  required  for  framed  trestles 
resting  on  mud-sills,  excavations  for  box-culverts,  baling  and 
pumping  out  water  after  rains,  and  many  things  that  may  arise 
during  the  construction  of  extensive  works.  It  is  true  that 
these  things  can  be  provided  for  in  the  contract,  but  however 
fully  and  carefully  the  contract  may  be  drawn  such  questions 
will  arise,  extra  bills  of  innumerable  kinds  will  be  presented, 
and  in  the  end  suits  will  be  brought  which  will  often  be  decided 
in  favor  of  the  contractor,  even  when  they  have  no  shadow  of 
a  just  claim.  The  contract  based  on  the  foot  of  length  is  open 
also  to  some  objections,  and  particularly  if  the  engineer  does 
not  know  by  careful  estimates  the  relations  between  the  costs 
of  the  actual  quantities  of  material  and  the  price  per  foot,  as 
the  contractor  will  certainly  on  his  part  put  the  cost  per  foot  at 
the  highest  possible  figure,  making  his  estimates  on  very  liberal 
allowances  for  quantities  and  contingencies. 

114.  In  order  to  avoid  waste  of  material  local  customs  should 


COST  OF  TIMBER  TRESTLES. 


245 


be  examined  into.  In  large  saw-mills  doing  a  regular  business, 
certain  definite  lengths  of  lumber  as  well  as  sizes  are  in  current 
demand,  either  for  local  use  or  for  shipment  to  different  and  dis¬ 
tant  places,  logs  are  cut  so  as  to  yield  these  lengths  and  sizes ;  and 
all  bills  of  lumber  that  cannot  be  fully  adjusted  to  these  will 
entail  either  extra  cost  at  the  mills  or  waste  in  the  works,  for 
which  the  company  will  have  to  pay.  If  the  common  run  of  the 
square  timber  and  plank,  scantling,  etc.,  is  in  lengths  of  even 
numbers,  such  as  12,  14,  16,  18,  and  20  ft.,  it  will  be  found 
economical  to  make  the  bill  of  lumber  for  any  particular  struc¬ 
ture  to  correspond  as  far  as  possible.  To  specify  that  the  posts 
of  a  trestle  bent  should  be  exactly  18  ft.  3^  in.  when  an  1 8-ft. 
post  would  do  as  well  is  simply  to  add  to  the  cost.  Where 
definite  lengths  must  be  obtained  it  cannot  be  helped.  Square 
timber  such  as  12  ins.X  12  ins.  is  used  for  stringers  by  some  en¬ 
gineers,  owing  to  the  difficulty  and  cost  of  obtaining  such  sizes 
as  6x  14  ins.  or  7X15  ins.  Either  using  shorter  spans  or  using 
built  beams  for  the  longer  ones,  or  as  before  mentioned  the  num¬ 
ber  of  pieces  can  be  increased,  thereby  decreasing  the  depths 
to  12  inches.  These  matters  are  merely  suggested  as  useful 
hints,  and  to  suggest  the  advantages  to  be  derived  from  allow¬ 
ing  slight  variations  in  designs,  rather  than  to  follow  some 
stereotyped  and  iron-clad  conditions  simply  because  somebody 
else  has  followed  them  before — always  bearing  in  mind  that 
strength,  suitableness,  and  durability  are  the  first  requirements  ; 
but  obtain  these  conditions  at  the  least  cost  and  in  the  least 
time. 

1 15.  It  is  often  necessary  to  cut  piles  off  below  water  sur¬ 
face  ;  this  may  be  required  at  any  depth  below  the  surface  from 
3  to  20  ft.  or  more,  as  when  cribs  or  open  caissons  are  to 
be  sunk  until  they  rest  on  the  piles.  There  are  three  methods 
of  doing  this,  1st.  By  the  use  of  professional  divers.  This  is 
an  expensive  and  slow  process,  as  at  best  they  can  work  only  a 
few  hours  a  day,  and  they  charge  high  for  their  services.  The 
diver’s  suit  consists  of  a  water-tight  canvas  suit  of  clothes, 
which  covers  and  fits  the  body  from  the  neck  to  the  ankles. 
Around  the  wrists  and  ankles  this  is  bound  tight  to  the  skin  by 


246  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


strong  rubber  bands.  Over  his  head  a  copper  helmet  is  placed, 
in  which  are  thick  glass  plates  called  bull’s-eyes  ;  this  fits  over 
his  shoulders  and  is  fastened  to  the  water-proof  suit  by  proper 
clamps,  rubber  bands  being  placed  between.  Connected  with 
this  helmet  is  a  long,  flexible  tube  or  hose,  which  connects  with 
the  helmet  at  one  end  with  a  valve  opening  inward,  and  at  the 
other  with  an  air-pump.  The  helmet  also  has  an  escape  valve 
for  foul  air  opening  outward.  The  bull’s-eyes  are  protected  by 
a  metal  netting  to  prevent  danger  of  breaking ;  these  should  have 
water-tight  valves,  which  the  driver  can  close  if  required.  To 
enable  him  to  sink  in  the  water  the  soles  of  the  shoes  are  made 
of  lead,  and  in  addition  lead  weights  are  fastened  to  his  breast 
and  back.  There  is  an  opening  in  the  helmet,  which  is  closed 
by  screwing  on  a  cap  just  as  the  driver  is  ready  to  descend- 
The  helmet  is  made  of  copper.  As  soon  as  the  cap  is  adjusted 
the  air-pump  must  be  started,  very  slowly  at  first,  but  more 
rapidly  as  the  diver  descends.  A  tender,  as  he  is  called,  holds 
the  hose  in  one  hand  and  a  rope  securely  tied  to  the  body  of 
the  diver  in  the  other,  and  he  pays  out  these  as  the  diver  de¬ 
scends  or  moves  about  on  the  bed  of  the  river.  It  requires  two 
men  at  the  pump — one  at  work  and  the  other  resting ;  these 
relieve  each  other  at  short  intervals,  and  they  should  turn  the 
crank  at  a  uniform  rate,  so  as  to  keep  a  constant  pressure  of  air 
in  the  helmet.  The  diver  signals  by  jerking  the  rope  once, 
twice,  or  three  times;  these  have  some  understood  meaning, 
such  as  more  air,  less  air,  or  to  lift  him  up,  and  so  on.  As  he 
rises  toward  the  surface  the  pump  is  worked  slower  and  slower, 
and  when  the  cap  is  removed  it  stops.  Divers  can  work  in 
depths  of  water  to  75  or  80  ft.,  but  only  for  a  very  short  time 
at  the  greater  depths.  Owing  to  the  cost  of  the  diver’s  ser¬ 
vices  piles  are  cut  off  by  saws  worked  by  machinery  from 
above. 

116.  A  simple  arrangement  for  this  purpose  is  to  fasten  a 
cross-cut  saw  to  the  bottom  of  a  frame  which  is  connected  to  a 
rod  suspended  from  a  bolt  attached  to  a  frame  constructed  on 
a  barge  ;  the  saw  being  adjusted  to  the  proper  depth,  a  swinging 
motion  is  imparted  to  it  by  men  on  the  barge  or  platforms  from 


COST  OF  TIMBER  TRESTLES. 


247 


above,  and  as  it  enters  the  pile  it  is  pressed  forward  by  a  lever  at¬ 
tached  to  the  bottom  of  the  lower  frame.  When  one  pile  is  cut 
off,  it  is  moved  to  the  next,  and  so  on.  Where  there  is  no  great 
current,  or  no  ebb  and  flow  of  the  tide  exists,  good  progress 
can  be  made  and  good  work  done  by  this  method.  While 
sawing  the  boat  must  be  kept  level,  unless  the  frame  above 
admits  of  the  suspending-rod  sliding  up  and  down.  Where  a 
large  number  of  piles  are  to  be  sawed  off,  the  following  arrange¬ 
ment  is  used  (see  Fig.  2  A,  Plate  V). 

117.  A  frame  is  constructed  on  a  barge,  or,  better,  a  floating 
pile  driver.  A  long  iron  shaft  carrying  a  horizontal  circular 
saw  is  so  suspended  and  connected  in  the  leads  that  it  can  be 
turned  and  at  the  same  time  raised  or  lowered  in  the  leads;  a 
band  wheel  or  drum  is  connected  with  the  shaft ;  the  power 
band  connects  this  with  the  drum  of  an  engine.  When  the 
power  is  applied  the  shaft  and  saw  are  made  to  revolve  rapidly. 
The  saw  is  adjusted  to  the  proper  depth,  and  started  ;  the 
pile  is  cut  off  in  a  few  seconds.  If  there  is  no  strong  cur¬ 
rent,  any  number  of  piles  can  be  cut  off  in  a  very  short  time. 
The  only  difficulty  in  a  current  arises  from  the  difficulty  of 
holding  the  barge  steady.  This  can  be  easily  controlled.  In 
a  tidal  stream  the  depth  of  the  saw  has  to  be  changed  more 
or  less  rapidly  as  the  tide  ebbs  and  flows.  To  regulate  this  an 
accurate  tide-gauge  must  be  placed  in  some  protected  place, 
where  it  can  be  easily  observed  either  by  the  foreman  on  the 
barge  or  by  an  assistant  ;  a  corresponding  scale  is  also  placed 
on  the  leads,  adjusted  to  the  plane  of  the  top  of  the  shaft,  or 
some  well-defined  mark  on  the  shaft.  The  reading  on  the 
scale  and  gauge  are  taken  simultaneously  at  the  commence¬ 
ment  of  the  sawing,  and  afterward  the  saw  is  raised  or  lowered 
^  in.,  ^  in.,  1  in.  from  time  to  time  as  the  tide  falls  or  rises.  With 
proper  care  and  precaution  a  large  number  of  piles  can  be 
cut  off  at  practically  the  same  elevation.  Upon  these  the 
opon  caisson,  or  crib,  or  other  structure,  can  be  lowered.  See 
Fig.  1  A,  Plate  V. 

118.  In  driving  piles  over  a  space  to  be  occupied  by  the 
structure,  the  outside  piles  should  be  driven  so  as  to  enclose  a 


248  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


space  several  feet  larger  than  the  actual  base  of  the  structure, 
or  equal  to  that  covered  by  the  platform  or  bottom  of  the 
caisson,  as  explained  in  paragraphs  20  and  21,  and  shown  in 
drawing  Figs.  1  and  2,  Plate  IV. 

119.  As  a  rule,  a  structure  thus  sunk  on  the  piles  simply 
rests  on  them,  the  weight  holding  it  in  place ;  and  although  it  is 
desirable  to  sink  such  a  structure  exactly  in  its  true  position,  a 
few  inches  one  way  or  another  out  of  line  or  distance  is  not  a 
matter  of  much  moment  in  most  cases,  as  the  masonry  required 
to  sink  it  should  be  a  few  inches  larger  than  actually  required  ; 
and  when  the  structure  is  finally  resting  firm  and  true,  an  offset 
can  be  made  on  the  top  of  the  masonry  so  as  to  place  the 
structure  in  its  true  line  and  distance.  The  little  excess  of 
masonry  thus  used  is  far  less  expensive  than  that  of  repeated 
raising  and  lowering  the  caisson.  And  unless  strong  staging 
is  constructed  around  the  caisson,  and  it  is  suspended  and 
lowered  by  long  rods  with  threads  and  nuts,  it  is  almost  im¬ 
practicable  to  lower  a  caisson  absolutely  in  a  desired  position. 
Such  staging  and  apparatus  are  expensive,  the  lowering  is  slow 
and  tedious.  If  necessary,  do  these  things;  but  when  not 
necessary,  such  useless  refinements  will  do  to  talk  about,  but 
must  be  paid  for  by  somebody.  Contractors  will  always  make 
the  company  pay  dearly  for  it.  The  driving  of  a  few  extra 
piles  is  far  better  and  less  costly,  a  small  margin  in  the  size  of 
the  platform  being  allowed.  This  is,  in  fact,  necessitated  by 
the  requirements  of  its  construction. 

120.  Cases  may  arise  where  it  becomes  necessary  to  pre¬ 
vent  any  tendency  to  slide  off  of  the  piles.  In  such  cases 
timber  strips  can  be  bolted  to  the  bottom  of  the  platform, 
projecting  downward  between  and  below  the  heads  of  the 
piles,  which  will  hold  the  structure  in  place.  And  in  some 
cases  iron  pipes  f  or  1  in.  in  diameter  are  built  in  the  caisson, 
extending  through  the  bottom  ;  and  when  the  caisson  finally 
rests  on  the  piles,  long  spikes  or  pointed  drift-bolts  can  be 
dropped  on  the  heads  of  the  piles  and  driven  into  them  by 
blows  from  above.  But  unless  a  grillage  is  constructed  with 
small  square  openings  in  it,  so  as  to  guide  and  hold  the  piles  in 


COST  OF  TIMBER  TRESTLES. 


249 


certain  positions,  the  pipes  would  be  as  likely  to  miss  the  piles 
as  to  rest  on  them.  Where  such  precautions  are  not  neces¬ 
sary  the  exact  positions  of  the  piles  are  not  of  much  moment  ; 
but  a  reasonable  effort  should  be  made  to  drive  them  in  rows 
at  specified  intervals,  such  as  2J  ft.  from  centre  to  centre,  and 
no  great  error  in  position  should  be  allowed.  This  cannot 
always  be  discovered  until  the  piles  are  cut  off,  as  when  freed 
from  wedges,  bars,  etc.,  they  are  apt  to  spring  more  or  less. 

121.  In  driving  piles  for  trestle-work,  it  is  important  that 
the  piles  in  each  bent  should  be  in  line  with  respect  to  each 
other,  and  also  that  the  piles  in  the  different  bents  should 
properly  line  up  with  each  other,  for  appearance  sake,  if  noth¬ 
ing  else.  The  difficulty  of  driving  piles  in  exact  line  is  doubt¬ 
less  very  great  and  often  impracticable,  but  it  can  be  done 
much  better  than  is  often  the  case ;  and  the  piles  have  to 
be  sprung  into  position  by  the  application  of  a  great  force. 
This  necessarily  bends  the  pile,  or  that  portion  of  it  above 
ground,  thereby  putting  it  in  an  unfavorable  position  to  carry 
heavy  vertical  loads.  Often  they  are  so  far  out  of  position 
that  they  cannot  be  sprung  or  pried  into  position,  and  conse¬ 
quently  the  cap  rests  on  only  about  one  half  of  the  pile. 
These  conditions  often  result  from  inexcusable  carelessness. 
Proper  care  is  not  taken  to  set  the  pile  or  to  hold  it  in  the  be¬ 
ginning  when  it  can  be  controlled,  but  after  it  has  penetrated 
to  a  considerable  distance  in  the  soil,  and  out  of  plumb  or 
position,  desperate  efforts  are  made  to  force  it  back,  which 
will  then  be  only  partially  successful,  if  at  all,  and  doubt¬ 
less  piles  are  seriously  crippled  or  even  broken  below  the 
ground  in  many  cases.  If  - the  pile  is  properly  pointed,  head 
cut  square,  and  set  straight,  and  properly  controlled  by  wedges 
■or  levers  until  it  penetrates  well  into  the  surface,  it  will 
be  easy  to  keep  it  straight  to  the  finish.  If  a  pointed  pile 
strikes  roots  or  even  small  bowlders  or  other  narrow  obstruc¬ 
tions,  it  will  inevitably  veer  out  of  position,  and  no  power  can 
prevent  it.  It  must  be  then  put  back  the  best  possible,  and  in 
some  instances  new  piles  have  to  be  driven.  As  previously 
mentioned,  it  is  easier  to  keep  a  blunt  pile  straight  than  a 


250  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


pointed  one.  In  alluvial  and  the  softer  soils  the  pile  should 
not  be  sharpened.  It  may  sometimes  be  necessary,  however, 
in  the  firmer,  stiffer  soils. 

122.  It  would  be  better  to  mark  with  a  peg  or  stake  the 
position  for  the  point  of  every  pile  ;  this  takes  time  and  labor 
and  the  dragging  and  lifting  of  piles  and  heavy  timbers  will  de¬ 
stroy  many  of  them,  but  enough  will  remain  to  prevent  any 
serious  error  in  alignment  or  position.  At  any  rate  a  peg 
should  be  driven  to  mark  the  centre  of  every  bent,  and  for  a 
few  bents  at  the  beginning  and  at  intervals  of  every  200  or 
300  feet  pegs  should  be  placed  for  every  pile.  By  this  means, 
the  piles  can  be  lined  by  sighting,  and  the  small  and  gradual 
errors  can  be  rectified  at  short  intervals,  and  by  fastening 
battens  at  intervals  on  the  piles  already  driven  the  leads  of  the 
driver  can  be  properly  lined  for  long  distances.  On  shore, 
with  set  lines  of  high  stakes  with  strips  of  paper  fastened  to 
them,  or  better  small  flags,  and  occasionally  placing  the  leads 
in  exact  position  with  the  transit,  a  true  line  can  be  kept  for 
long  distances.  This  should  always  be  done  when  driving 
across  water. 

123.  It  is  better  not  to  drop  a  pile  after  being  lifted  be¬ 
tween  the  leads,  unless  it  is  so  long  that  this  is  necessary  to 
get  the  head  under  the  hammer,  as  it  is  difficult  to  drop  it  in 
the  exact  position  or  in  a  vertical  line.  Sometimes  it  is  neces¬ 
sary  owing  to  its  length  to  drop  it  in  front  of  the  leads,  so  as 
to  let  it  penetrate  as  far  as  possible  into  the  soil,  and  then 
move  the  driver  forward  ;  this  requires  great  care. 

124.  During  the  driving  the  direction  of  the  pile  is  con¬ 
trolled  by  short  blocks  of  wood,  wedges,  and  levers.  The  leads 
of  the  drivers  have  iron  brackets  bolted  to  them,  which  hold 
the  blocks  of  wood.  The  pile  is  lifted  in  the  leads,  lowered 
and  set  in  position  at  its  point,  forced  into  a  vertical  position, 
and  the  blocks  are  then  placed  in  front  and  rear  and  wedged 
into  position.  This  is  done  at  one  or  two  points  in  its  height;: 
then  the  driving  commences:  the  wedges  are  loosened  or  tight¬ 
ened  so  as  to  keep  the  pile  vertical,  or  these  are  omitted,  and 
the  piles  held  and  controlled  by  levers  handled  by  the  men. 


EMBANKMENTS  OF  EARTH  ON  SWAMPS.  2$I 

This  imposes  very  hard  work  on  the  men  during  the  entire  time 
of  driving.  Either  plan  can  be  used,  but  the  first  seems  to  be 
preferred.  In  stiff,  compact  silt,  or  ordinary  clay  it  will  be 
found  convenient  to  drive  a  short  pile,  which  is  then  pulled 
out,  and  the  longer  pile  let  down  into  the  hole  to  a  depth  suf¬ 
ficient  to  bring  the  head  of  the  pile  under  the  hammer.  Long, 
heavy  piles  can  be  set  more  accurately  in  this  way  than  by 
dropping  them  in  front  of  the  leads. 


Article  XLVI. 

EMBANKMENTS  OF  EARTH  ON  SWAMPS. 

125.  As  has  been  mentioned,  timber  trestles  are  to  a  large 
extent  temporary  structures,  and  it  is  expected  to  substitute 
iron  trestles  or  embankments  of  earth  sooner  or  later.  This  is 
also  applicable  to  a  considerable  extent  in  building  roads  across 
extensive  swamps ;  but  here  it  must  not  be  lost  sight  of  that 
the  rises  in  the  rivers  and  streams  intersecting  them,  and  the  flow 
of  the  tides,  especially  in  cases  of  storms,  cover  these  swamps 
to  the  depth  of  3  to  6  ft.,  and  that  ample  water-way  must  be 
provided.  Therefore  long  stretches  of  trestle  are  necessary,, 
which  will  constitute  permanent  important  parts  of  the 
work.  With  this  precaution  it  is  intended  to  ultimately  form 
earth  embankments  when  material  for  the  same  can  be  secured 
in  the  necessary  quantities  and  at  a  reasonable  cost,  and  after 
the  road  has  been  constructed  the  construction  trains  can 
gradually  dump  dirt  under  and  around  the  trestles  until  the 
embankments  are  completely  formed.  This  will  require  a  large 
amount  of  material,  as  the  weight  of  the  earth  breaks  through 
the  matting  or  crust  of  roots  and  sinks  to  an  unknown  depth, 
but  ultimately  it  will  cease  to  settle,  and  a  permanent  embank¬ 
ment  takes  the  place  of  the  trestles. 

126.  The  matting  of  roots,  of  the  cane  and  undergrowth 
that  grow  so  largely  in  these  swamps,  has  sufficient  strength 
to  carry  the  weight  of  two  or  three  feet  of  earth  and  a  light 
construction  engine  and  dump-cars ;  but  this  is  its  ultimate 


2$2 


A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


strength.  Any  increase  of  weight  will  break  through.  When, 
therefore,  earth  can  be  obtained  from  the  neighboring  eleva¬ 
tions,  these  are  staked  out  for  borrow-pits,  and  tracks  are 
laid  from  them  to  connect  with  the  track  of  the  main  line 
of  the  road.  The  cross-ties  are  laid  directly  on  the  swamp. 
Trains  loaded  with  earth  are  then  run  on  to  the  main  track, 
the  dirt  dumped  on  the  swamp,  and  gangs  of  men  raise  the 
track  with  levers ;  the  earth  is  thrown  and  rammed  under 
the  ties.  When  this  embankment  reaches  a  height  of  two 
to  three  feet,  the  crust  breaks  short  off  along  the  foot  of 
the  embankment ;  the  embankment  and  track  settle  into  the 
liquid  mud  underlying  the  crust.  As  fast  as  the  material  can 
be  added  it  settles  down;  embankments  several  feet  high  in  the 
evening  will  entirely  disappear  by  the  following  morning,  only 
to  be  filled  again.  This  may  continue  for  weeks,  gradually 
settling  more  and  more  slowly,  until  finally  it  will  practically 
cease,  but  in  a  greater  or  less  degree  will  continue  for  months 
or  years. 

127.  The  depth  to  which  this  will  reach  below  the  swamp 
is  probably  not  known,  but  must  be  very  great — not  less  than 
10  to  15  ft.  This  conclusion  was  reached  by  the  writer  in  ob¬ 
serving  the  effect  upon  the  swamp  on  either  side  of  the  em¬ 
bankment.  The  swamp  bulges  up  fully  6  ft.  on  either  side, 
somewhat  abruptly  facing  the  bank,  and  sloping  rather  gently 
from  this  summit  outward  to  the  level  of  the  swamp,  the 
crust  forming  over  the  mound  a  smooth,  uniform  covering,  this 
mass  of  material  representing  the  displacement  made  by  the 
earth  of  the  embankment.  The  earth  doubtless  assumes  a 
slope  considerably  steeper  than  its  natural  slope,  probably  not 
more  than  ^  to  £  to  1,  which  would  fully  justify  the  depth 
above  stated.  After  the  lapse  of  time  the  mound  settles  down 
to  the  general  level  of  the  swamp ;  this  has  been  observed 
for  miles.  As  a  further  proof  of  the  great  fluidity  of  the  ma¬ 
terial  and  the  depth  to  which  the  earth  sinks,  the  effect  upon 
the  trestle  approaches  of  bridges  over  the  bayous  and  streams 
intersecting  these  swamps  will  be  mentioned.  The  approaches 
to  these  bridges  were  built  of  pile  trestles  in  lengths  from  25 


EMBANKMENTS  OF  EARTH  ON  SIVAMPS. 


253 


to  IOO  ft.,  and  as  the  earth  embankment  was  built  up  to  the 
ends  of  these  and  pressed  against  the  end  piles,  the  entire 
trestle  would  be  pushed  forward,  and  this  also  pushing  the 
abutments  against  the  ends  of  the  draw  bridge  so  firmly  as 
to  prevent  the  draw  from  opening  until  the  latch  beams  were 
moved  backward  ;  and  this  was  repeated  many  times.  The 
material  was  so  soft  that  in  walking  on  the  swamp  and  failing 
to  plant  the  foot  on  the  roots  of  the  cane  a  man  would  sink  to 
his  waist  before  getting  support.  Such  was  the  material  upon 
which  14  miles  of  road  was  constructed,  and  into  which  piles 
driven  from  30  to  40  feet  would  support  load.  The  material 
for  such  banks  should  be  sand  and  gravel  or  sand  alone ; 
clayey  soils  would  be  apt  to  form  mud,  and  be  but  little  firmer 
than  the  swampy  material  itself. 

128.  Sometimes  a  layer  of  long  logs  or  plank  is  first  laid 
on  the  swamp  so  as  to  give  a  broad  base  for  the  embankment, 
and  if  broad  enough  it  would  keep  the  crust  on  the  surface 
from  breaking  through  ;  this  answers  well  for  support,  but  is 
probably  wanting  in  steadiness,  and  a  rapidly  moving  train 
tends  to  produce  a  wave-like  motion.  It  is  more  economical 
than  the  first  method,  but  cannot  be  considered  as  good  or 
as  safe. 

129.  These  methods  of  embanking  are  very  objectionable,  as 
the  track  has  to  be  raised  as  the  earth  is  packed  under  the  ties. 
The  result  is  that  the  rails  are  badly  sprung  or  bent,  both  in  a 
vertical  and  a  horizontal  plane  ;  the  former  being  more  objec¬ 
tionable  than  the  latter,  as  the  horizontal  bends  are  the  more 
easily  seen  and  removed  in  part,  if  not  entirely.  A  temporary 
trestle  consisting  of  two  short  piles  could  be  constructed  of 
the  proper  height ;  this  would  carry  a  light  train,  and  the 
earth  embankment  could  be  formed  under  and  around  it.  It 
would  cost  something,  but  at  any  rate  would  save  the  rails. 
It  is  the  plan  adopted  even  on  firm  ground,  where  the  ma¬ 
terial  is  hauled  out  by  engine  and  cars.  The  trestles  in  this 
case  are  framed  and  a  light  rail  is  used.  The  cost,  however, 
would  be  the  same  in  the  two  cases,  which  would  be  practically 


254  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


offset  by  the  extra  labor  required  in  rehandling  the  earth  and 
raising  the  track. 

130.  A  few  general  remarks  on  earth-work  will  be  made  in 
this  connection.  The  earth-work  on  a  line  of  road  consists  of 
embankments  and  excavations.  After  locating  the  line  of  the 
road  and  establishing  the  grade  line  the  road  is  divided  into 
sections,  somewhat  in  an  arbitrary  manner — the  length  of  the 
sections  being  so  regulated  that  the  material  excavated  may  be 
sufficient  to  make  the  embankments  within  the  limits  of  the 
section,  or  for  some  other  reason,  the  average  length  o.f  the 
sections  being  from  one  to  two  miles.  The  earth  from  the 
excavation  is  hauled  by  barrows,  carts,  horse-cars,  or  by  the  use 
of  a  locomotive  engine,  depending  on  the  amount  of  work, 
length  of  haul,  etc.  Sometimes  it  is  more  economical  to  waste 
the  material  from  the  excavation  on  the  sides  or  at  the  ends  of 
the  cut,  and  to  make  the  embankments  from  trenches,  ditches, 
or  borrow-pits  along  the  embankment ;  these  matters  are  regu¬ 
lated  by  considerations  that  will  not  be  discussed  in  this 
volume.  Ditches  along  the  embankments  are  necessary  for 
purposes  of  drainage,  and  should  be  cut  as  straight  and  as 
regular  as  possible.  A  space  called  the  berm  should  be  left 
between  the  foot  of  the  embankment  and  the  ditch  ;  the  width 
of  this  space  is  regulated  so  that  the  prolongation  of  the  plane 
of  the  slope  of  the  embankment  shall  pass  well  under  the 
bottom  of  the  ditch  on  the  berm  side,  which  will  require  a  berm 
of  from  three  to  six  feet,  according  to  the  depth  of  the  ditch. 
The  side  of  the  ditch  should  have  a  slope  whose  base  is  equal 
at  least  to  its  depth,  so  as  to  prevent  caving  in.  The  cuts  are 
excavated  so  that  the  slopes  will  be  one  vertical  and  one  hori¬ 
zontal,  or  one  to  one  as  it  is  called,  and  the  width  at  bottom 
varies  for  a  single  track  from  16  to  18  ft.,  so  as  to  allow 
for  side  drains.  The  width  of  the  embankments  on  top  vary 
from  12  to  14  ft.,  and  the  side  slopes  one  and  one  half  hori¬ 
zontal  to  one  vertical.  Whether  the  embankments  are  made 
from  cuts  or  trenches,  they  should  always  be  started  with 
the  full  width  required  at  the  bottom  and  maintained  the  full 
width  to  the  top.  A  too  common  practice  is  to  make  a  narrow 


EMBANKMENTS  OF  EARTH  ON  SWAMPS. 


255 


core  at  first,  and  then  to  widen  it  out  by  dumping  loose  earth 
-on  the  sides ;  the  core  being  hauled  over  settles  and  compacts; 
the  loose  earth  thrown  on  the  sides  sloughs  off,  and  will  not  bond 
with  it.  And  again,  in  making  the  filling  a  too  common  habit  is 
to  keep  the  embankment  higher  at  the  centre  than  at  the  sides. 
This  is  just  the  reverse  of  what  it  should  be.  Each  layer  should 
be  a  fraction  lower  in  the  centre.  This  rule  should  always  be 
observed  when  the  embankment  is  made  in  layers.  Embank¬ 
ments  made  from  cuts  are  generally  built  in  one  thick  layer,  of 
the  required  height  of  the  fill ;  this  is  done  by  dumping  the 
earth  at  the  end  of  the  embankment.  The  practice  is  still  to 
keep  the  bank  too  narrow  ;  it  should  be  built  of  the  full  width 
from  bottom  to  top.  Broken  stone,  gravel,  sand,  or  mixed 
earths  make  the  best  embankments.  Clay  makes  a  good  em¬ 
bankment  when  put  up  dry  and  properly  drained.  All  earth 
embankments  will  settle  more  or  less,  depending  upon  the 
character  of  the  material  used,  and  the  manner  in  which  the 
-embankment  is  constructed.  Clay  and  ordinary  earth  settle 
slowly,  and  to  a  considerable  extent,  and  more  than  sand  or 
gravel.  It  is  not  unusual  to  allow  as  much  as  ten  per  cent  for 
settlement  ;  that  is,  a  bank  ten  feet  high  must  be  made  eleven 
feet  high  on  first  construction.  Low  embankments  that  are 
made  either  by  throwing  the  material  from  trenches,  or  by  the 
use  of  barrows  will  require  the  full  allowance  for  settlement. 
High  embankments  constructed  by  the  use  of  scoops  or  drags 
drawn  by  horses,  by  horse-carts,  and  by  engines  and  dump-cars, 
owing  both  to  the  time  required  in  the  construction,  and  also 
to  the  constant  tramping  and  hauling  over  them,  will  settle  to 
a  large  extent  during  construction,  and  will  require  but  a  small 
per  cent,  of  additional  height.  The  slopes  of  the  cuts  are  liable 
to  be  washed  into  gulleys,  and  undermined  by  the  flow  of  sur¬ 
face  water  running  down  the  slope,  or  sinking  into  the  soil  and 
escaping  along  seams  or  through  porous  layers  of  sand  or 
gravel.  This  can  be  greatly  reduced  by  cutting  surface  drains 
on  the  up-hill  side  of  the  cut,  or  by  surface  drains  on  the  slope 
made  of  timber,  or  by  terra-cotta  pipes  imbedded  in  the  slope, 
and  emptying  in  the  side  drains  at  the  bottom.  Sodding  the 


256  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 

slopes  or  sowing  grass-seed  on  them  is  also  a  remedy  for  this 
trouble,  besides  adding  greatly  to  the  appearance.  In  some 
cases  these  methods  fail,  and  the  slopes  will  cave  in.  In  this 
case  benches  can  be  cut  at  different  elevations,  so  as  to  break 
the  slope  ;  drains  made  on  the  benches  will  carry  off  the  water. 
Foot  walls  can  be  constructed  of  masonry  at  the  bottom  of  the 
slope  ;  even  when  very  thick  they  often  prove  of  little  value. 
All  of  these  means  failing,  the  material  can  only  be  removed  as. 
it  falls.  If  the  weight  and  amount  of  traffic  were  the  same  in 
both  directions  a  straight  and  level  line  would  be  desirable;, 
but  in  the  direction  of  the  heaviest  traffic  gentle  inclines  or 
grades  are  advantageous,  as  they  aid  in  hauling  long  and  heavy 
trains,  and  are  of  no  serious  obstruction  on  the  return  with  the 
lightly  loaded  or  empty  cars.  Grades  also  facilitate  the  drain¬ 
age  of  the  road-bed.  Grades  vary  from  o  to  2  ft.  per  100  ft.  ; 
the  usual  grades,  however,  vary  from  20  to  52.8  ft.  per  mile; 
they  should  not  be  used  on  curves,  trestles  or  bridges  when 
not  absolutely  necessary. 

131.  The  roadbed  completed,  when  it  can  be  economically 
done,  on  it  should  be  placed  the  ballast,  which  consists  of  a 
layer  of  gravel  or  broken  stone  from  6  to  9  inches  thick,  upon 
which  the  cross-ties  are  laid  ;  then  between  the  cross-ties 
gravel  or  broken  stone  should  be  packed.  The  ballast  gives- 
firmness  to  the  bed  and  serves  also  to  drain  off  the  water, 
thereby  keeping  the  track  dry,  and  consequently  preserving 
the  ties.  As  a  rule,  however,  the  ties  are  first  laid  on  the 
earthen  embankment,  the  rails  laid,  and  the  ballasting  done 
afterward,  when  it  can  be  hauled  in  construction  trains  and 
distributed  more  economically.  The  track  is  raised,  and  the 
ballast  then  rammed  under  the  ties  and  between  them,  as  be¬ 
fore.  When  broken  stone  is  used  the  ballast  is  built  up  to  the 
top  of  the  tie  for  its  full  length,  with  the  proper  slope  on  the 
sides.  Sand  and  gravel  also  make  good  ballast ;  but  sand,  espe¬ 
cially  if  very  fine,  makes  a  dusty  road,  and  the  grit  deposits  in 
the  machinery,  which  causes  friction  and  wear.  Sandstone  is 
apt  to  be  pulverized,  and  has  the  objection  just  mentioned. 
In  many  sections  of  the  country  broken  stone  of  any  kind  is 


EMBANKMENTS  OF  EARTH  ON  SWAMPS.  257 

hard  to  obtain,  and  the  dirt  ballast,  so  called,  is  used  This 
simply  means  packing  the  dirt  under  and  between  the  ties,  so 
as  to  give  firmness  to  the  track.  In  this  case  drainage  is  pro¬ 
vided  by  simply  sloping  the  top,  so  that  the  surface  at  the 
centre  of  the  track  is  level  with  the  top  of  the  tie  and  slopes 
gently  on  each  side,  so  as  to  fall  to  the  level  of  the  bottom  of 
the  ties  at  their  ends.  The  water  is  thereby  drained  off.  But 
this  kind  of  ballast  is  apt  to  work  into  the  condition  of  mud 
during  very  wet  weather  by  the  churning  motion  imparted  to 
the  ties  by  a  rapidly  moving  train.  But  it  is  the  only  kind  of 
ballast  used  on  thousands  of  miles  of  road  in  the  Southern  and 
Western  States.  It  is  not  favorable  for  very  heavy  loads  or 
very  high  speeds. 

132.  Cross-ties  are  placed  generally  at  intervals  of  2  to  2 % 
ft.  centres;  requiring  from  2640  to  2112  ties  to  the  mile.  The 
depth  of  the  tie  is  from  6  to  7  ins.,  the  width  from  8  to  10 
ins.,  and  length  from  8  to  9  ft.,  8^  ft.  being  about  the  average. 
These  ties  are  hewn  on  two  sides  and  the  bark  stripped  off 
the  other  two.  It  is  usual  to  place  the  ties  somewhat  closer 
together  at  the  joints,  these  being  between  the  ties  form  the 
suspended  joint.  In  the  other  case  the  joint  rests  on  the  tie, 
a  broad  cross-tie  being  selected  for  this. 

Cross-ties  are  generally  made  of  white  oak,  post  and  chest¬ 
nut  oak,  white  or  yellow  pine,  and  sometimes  of  other  woods. 

133.  The  cost  of  the  earth  work  varies  somewhat,  but,  as  a 
rule,  the  established  price  is :  Earth,  16  to  20  cts. ;  hardpan,  30  to 
35  cts. ;  soft  or  loose  rock,  40  cts.,  and  hard  rock  in  large  masses, 
80  cts.  per  cubic  yard,  and  when  the  material  has  to  be  hauled 
more  than  a  certain  specified  distance  an  extra  allowance  is 
made,  such  as  |  or  I  cent  per  cubic  yard  for  each  hundred  feet 
of  haul  over  300  or  500  ft.  Disputes  often  arise  on  this  point 
on  the  final  settlement  from  the  indefinite  manner  in  which 
this  is  expressed  in  the  contract. 

134.  Cross-ties  vary  in  cost  from  20  to  50  cts.  apiece, 
depending  upon  whether  pine  or  oak  is  used,  and  upon  the 
more  or  less  abundance  of  the  timber  suitable  for  ties  along 
the  line  of  the  road.  These  may  be  taken  as  extremes,  the 


258  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


average  prices,  delivered  and  piled  at  intervals  along  the  road, 
being  30  and  40  cts.  respectively.  Piles  of  ties  should  be 
formed  by  first  laying  two  or  three  ties  with  intervals  on  the 
ground,  then  a  solid  layer  at  right  angles  to  these,  then  a  layer 
of  two  or  three,  and  another  solid  layer,  and  so  on  to  the 
height  of  a  man’s  head.  This  mode  of  piling  enables  the  ties 
to  be  easily  inspected,  favors  the  gradual  seasoning  of  the  tie, 
and  adds  greatly  to  the  life  of  the  tie.  Solid  or  irregular  piles 
of  ties  cannot  be  properly  inspected.  The  general  rule  is  to 
require  the  ties  to  be  hewn  to  smooth  surfaces  on  top  and  bot¬ 
tom.  Careless  work  leaves  gashes  in  the  tie  that  admit  and 
hold  water,  thereby  hastening  rot.  One  end  of  the  tie  is 
required  to  be  cut  square,  though  the  other  may  not  be.  The 
ties  are  laid  with  the  square  ends  on  a  lme  parallel  to  the  centre 
line  of  the  road. 

135.  On  a  road  in  Central  America  the  writer  used  lignum 
vitae  and  mahogany  ties.  This  is  mentioned  more  as  an  illus¬ 
tration  of  the  use  of  that  particular  kind  of  material,  which 
grows,  regardless  of  its  intrinsic  value,  in  any  particular  lo¬ 
cality,  and  which  is  often  carried  to  the  opposite  extreme  by 
using  very  inferior  materials  on  account  of  the  convenience 
and  cheapness  of  obtaining  them.  Lignum  vitae  would  doubt¬ 
less  be  an  economical  tie  in  the  end,  no  matter  what  its  first 
cost,  but  it  cannot  be  obtained  in  large  quantities.  The  life 
of  a  tie  depends  greatly  on  the  use  of  good  ballast  and  also 
upon  its  resistance  to  being  cut  into  by  the  rail,  as  this  induces 
rot  and  loosens  the  hold  of  the  spike.  This  peculiar  property 
is  marked  in  the  ties  of  the  lignum  vitae  kind. 

136.  As  has  been  mentioned,  the  two  proper  and  usual 
methods  of  building  across  swamps  are  first  to  dump  the  earth 
directly  on  the  swamp,  and  continue  doing  so  until  settling  prac¬ 
tically  ceases,  or  to  prevent  settling  by  floating  platforms  of 
plank,  fascine-mattresses,  or  logs;  and  secondly,  to  drive  piles 
and  fill  in  to  a  great  or  less  extent  subsequently  with  earth. 
Either  of  these  plans  is  good,  but  there  have  been  fatal  and 
serious  blunders  in  building  across  swamps,  by  cutting  canals 
with  dredges  along  the  road-bed  and  emptying  the  material  on 


EMBANKMENTS  OF  EARTH  ON  SWAMPS.  259 

the  road-bed.  This  is  bad  practice,  for  several  reasons,  1.  It 
cuts  or  breaks  the  crust  formed  by  the  matting  of  roots,  which 
is  the  main  reliance  to  prevent  excessive  settling  and  sinking 
of  the  bank.  This  has  been  done,  and  to  remedy  the  blunder 
double  rows  of  piles  have  been  driven  along  the  foot  of  the 
slope  to  hold  the  bank,  but  this  failed — as  this  swampy  mate¬ 
rial,  especially  when  the  crust  is  broken,  has  but  little  stability 
so  far  as  lateral  resistance  is  concerned,  and  the  piles  would 
spread  outward  at  and  near  the  top.  The  writer  saw  the 
above  conditions  on  the  road  between  New  Orleans  and 
Mobile.  The  subsequent  labor  and  cost  of  securing  a  firm 
road-bed  must  have  been  enormous. 

2.  The  material  of  the  swamp,  even  if  it  can  be  held  in 
place,  is  in  the  writer’s  opinion,  unfit  for  use  in  an  embank¬ 
ment.  There  is  now  being  constructed  a  road-bed  across  these 
same  swamps,  in  which  an  attempt  is  being  made  to  do  away 
with  the  first  difficulty  by  cutting  the  canal  at  a  distance  of 
50  to  150  feet  from  the  road-bed.  Whether  this  will  be  effect¬ 
ive  and  fully  remove  the  difficulty,  probably  is  yet  to  be  de¬ 
termined.  The  material  thus  dredged  is  hauled  and  used  in 
the  embankment ;  the  second  difficulty  then  still  exists,  and  it 
is  doubtful  whether  such  an  embankment  will  ever  prove 
satisfactory.  It  is  probable  that  a  temporary  trestle,  built 
with  two  pile  bents,  and  subsequently  filled  in  with  some  more 
stable  material,  such  as  sand  or  gravel,  would  prove  ultimately 
more  economical  and  satisfactory.  The  method  is  certainly 
an  improvement  as  compared  with  the  preceding  one  just 
described. 

An  interesting  instance  of  this  subject  is  the  observed  set¬ 
tling  of  the  peaty  soil  in  Holland.  An  embankment  of  sand 
8  ft.  high,  giving  a  load  at  base  of  800  lbs.  per  square  foot, 
compressed  the  peat  underneath  to  two  thirds  its  bulk ;  final 
condition  of  stability  was  only  attained  after  two  years.  The 
embankment  afterward  carried  safely  the  railroad  trains.  The 
soil  was  covered  with  a  fascine-mattresses,  to  distribute  the  pres¬ 
sure  and  prevent  the  sand  sinking  into  the  soft  silt.  In  con¬ 
structing  a  station  yard,  a  trench  15  ft.  wide  and  15  ft.  deep 


260  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 

was  excavated  around  the  space  and  filled  with  sand,  and  a 
3-ft.  layer  of  sand  spread  over  the  entire  space ;  and  for  the 
station  building  itself  a  pit,  15  ft.  deep  and  of  horizontal  di¬ 
mensions  each  way  20  ft.  greater  than  that  of  the  structure, 
was  excavated  and  filled  with  sand  ;  the  sand,  as  has  been 
mentioned,  distributing  the  pressure  over  the  entire  surface  of 
the  excavation. 

The  theoretical  discussion  of  the  stability  of  earth  under 
pressure  as  found  in  Rankine  is  very  pretty,  even  if  not  true, 
and  easily  reduces  to  the  well-settled  theory  of  fluid  pressure. 
In  Fig.  6,  ABCD  is  the  cross-section  of  the  excavation  in  the 


E 


soft  material,  and  AEFBDCA  is  a  cross-section  of  the  pro¬ 
posed  embankment  or  other  structure  to  be  built.  Making 
GE  —  h,  GC  =  It' ,  0'  =  angle  of  repose  of  soft  material,  w'  — 
weight  of  one  cubic  foot  of  the  soft  material,  w  —  weight  of  a 
cubic  foot  of  the  sand,  gravel,  or  stone  used  in  the  structure, 
I  —  sin  <p' 

k’  =  — - — : — -77.  Then  GC ,  the  depth  to  which  the  pit  is  to 
1  -(-  sin  0  r 

be  excavated,  =  h’  = 


Jnvk1' 


,  ,n,  for  a  fluid  0'  =  o,  and  the 

W  —  wk 

formula  becomes,  since  k!  =  1, 


h'  = 


hw 


w 


■  w 


or  zv  (It  -f-  It')  =  w'h' , 


which  simply  means  that  the  structure  will  sink  until  the 
weight  of  the  displaced  soil  is  equal  to  the  weight  of  the  struct- 


EMBANKMENTS  OF  EARTH  ON  SWAMPS.  26 1 

ure  itself,  which  is  the  law  of  fluid  pressure  ;  for  semifluids  or 
firmer  earths  this  is  modified  by  the  value  of  <p' ,  the  angle  of 
repose  of  the  material.  Such  formulae  should  be  of  course 
used  with  precaution  and  a  large  factor  of  safety,  and  can 
only  be  regarded,  as  was  mentioned  in  discussing  the  subject 
of  retaining  walls,  as  a  very  ingenious  and  masterly  extension 
of  the  theory  of  fluid  pressure  to  that  of  earth  pressure. 

Those  cases  in  which  a  firm  stratum  is  underlaid  by  a  soft 
material,  such  as  mud  or  quicksand,  involving  as  they  do  many 
difficulties  and  requiring  special  methods  of  construction,  will 
be  classed  under  the  head  of  difficult  foundations,  and  will  be 
discussed  in  the  next  part  of  this  volume.  And  similarly, 
where  the  soft  material  overlies  a  firmer  stratum,  where  piles 
are  not  used,  although  there  are  but  few  such  cases  in  which 
piles  would  not  answer  every  purpose,  and  in  general  be 
economical ;  but  often  they  would  be  unsuitable,  expensive, 
and  undesirable. 


PART  THIRD. 


Article  XLVII. 

FOUNDATIONS— (CONTINUED). 

DEEP  FOUNDATIONS. 

I.  Having  considered  what  may  be  called  ordinary  founda¬ 
tions,  including  timber  trestles  and  pile  trestles,  and  in  part 
first  masonry  and  masonry  piers  from  the  foundation-beds 
to  the  bridge  seats,  we  will  now  explain  and  discuss  those 
foundations  requiring  more  costly  and  difficult  methods  of  con¬ 
struction.  For  convenience,  foundations  were  divided  into  two 
parts,  that  portion  from  the  foundation-bed  reaching  to  or  nearly 
to  the  surface  of  the  ground,  and  that  portion  above  and  extend¬ 
ing  to  the  bottom  of  the  superstructure  ;  these  together  are 
commonly  known  as  the  substructure.  To  complete  this  por¬ 
tion  of  the  subject,  it  only  remains  to  describe  certain  unusual 
methods  of  reaching  the  foundation-bed,  where  great  depths 
below  the  water  or  earth  surfaces  have  to  be  reached.  These 
methods,  disregarding  the  materials  used,  which  may  be  either 
wood  or  iron  or  both  combined,  may  be  divided  into  two  classes. 
1st.  Where  the  desired  depth  is  reached  by  simply  dredging  the 
material  from  the  interior  of  a  large  timber  or  iron  box  or 
cylinder,  suitably  and  strongly  constructed  for  the  purpose, 
and  forcing  the  structure  to  sink  against  the  exterior  friction 
on  its  sides,  by  sufficient  weights  or  loads  superimposed. 
Structures  of  this  class  are  called  either  open  caissons,  or  more 
commonly  cribs.  2d.  Those  methods  in  which  timber  or  iron 
boxes  or  cylinders  are  constructed  with  one  or  more  air-tight 
compartments,  except  that  they  are  open  at  the  bottom. 

262 


THE  OPEN  CRIB. 


263 


This  part  of  the  structure  is  called  a  pneumatic  caisson,  and 
upon  this  can  either  be  constructed  cribs  of  a  greater  or  less 
height,  on  which  the  masonry  rests,  or  this  latter  may  rest 
directly  upon  the  roof  or  deck  of  the  caisson. 

2.  The  first  or  crib  method  will  now  be  considered.  When 
constructed  of  timber,  the  crib  is  composed  of  four  double 
walls  of  timber,  enclosing  a  space  of  the  proper  horizontal  and 
vertical  dimensions;  the  two  walls  of  each  side  maybe  built 
solid  and  connected  together  by  horizontal  struts  and  ties,  or 
they  may  be  built  somewhat  open  and  similarly  connected. 
These  walls  near  the  bottom,  and  for  a  varying  height,  are  con¬ 
structed  with  V-shaped  sections,  coming  together  at  the  bot¬ 
tom  edge,  thereby  forming  a  cutting  edge,  and  opening  out 
gradually  to  a  width  of  8  or  10  ft.,  at  a  height  of  about  8  or 
10  ft.  The  outer  wall  has  a  batter  or  slope  outward  and 
downward,  varying  from  a  few  inches  to  several  feet,  the 
inner  wall  is  constructed  to  a  slope  of  450  or  less.  This 
lower  section  of  the  crib  may  be  built  solid  with  large  timbers, 
or  it  may  only  be  strongly  braced  with  cross-timbers.  Upon 
the  top  of  this  bottom  section  the  two  walls  are  built  up  verti¬ 
cally  and  parallel,  or  the  outer  wall  may  have  a  slight  batter 
of  about  one  half  inch  to  the  vertical  foot,  and  the  two  prop¬ 
erly  tied  together.  The  object  of  the  space  between  the  two 
walls  is  to  give  strength  and  stiffness  to  the  sides  of  the  crib 
and  at  the  same  time  to  supply  sufficient  space  for  the  weight 
required  to  sink  the  crib,  which  weight  is  generally  either 
gravel,  broken  stone,  or  concrete.  For  cribs  enclosing  small 
areas  the  outer  walls  thus  filled  are  all  that  are  necessary.  In 
large  cribs,  however,  cross-partitions,  similar  to  the  enclosing 
walls,  are  constructed.  One  longitudinal  partition  will  ordi¬ 
narily  be  sufficient,  but  there  may  be  several  transverse  parti¬ 
tions.  This  construction  divides  the  enclosed  space  into 
several  square  or  rectangular  divisions,  open  top  and  bottom. 
See  Plates  XI  and  XII,  Figs.  I,  2,  3,  and  4. 

3.  A  sufficient  height  of  crib  being  built,  either  floating  or 
on  land  and  then  launched,  the  crib  is  then  floated  and  anchored 
over  the  proposed  site  of  the  pier  and  held  in  position  by  clusters 


264  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


of  piles,  anchors,  etc.  The  building  is  continued  until  the  crib 
rests  on  the  bed  of  the  river  or  sinks  some  distance  into  it. 
Then  the  work  of  removing  the  material  on  the  interior  is 
commenced.  Many  more  or  less  crude  means  of  doing  this 
have  been  practiced,  such  as  ordinary  scoops  or  iron  buckets, 
connected  to  and  worked  by  suitable  gearing  and  machinery 
on  top  of  the  crib  itself,  or  resting  on  platforms  or  barges. 
But  now  some  form  of  clam-shell  dredge  is  generally  used. 
This  may  be  defined  as  a  large  bucket,  composed  of  several 
sections  so  hinged  and  connected  that  when  it  descends  the 
sections  separate,  and  its  weight  forces  itself  into  the  soil  or 
around  and  over  bowlders;  when  lifted,  the  segments  close 
together  on  the  material,  which  is  then  lifted  to  the  surface 
and  either  emptied  into  the  river  or,  when  necessary,  into 
barges.  While  the  material  is  being  dredged  out  the  crib  is 
built  up,  the  pockets  filled  with  the  stone  or  concrete.  With 
the  relief  from  resistance  on  the  interior  and  the  weight  of 
the  structure,  the  crib  sinks  into  the  material  at  the  bottom, 
either  gradually  and  continuously  or  at  intervals,  depending 
on  the  resistance  and  weight.  The  method  is  simple  and  was 
formerly  resorted  to  for  great  depths,  either  where  it  was  not 
desired  to  drive  piles  or  where  the  ordinary  coffer  dams  were 
either  unfit  or  too  costly.  For  depths,  say  from  30  to  100  feet, 
the  pneumatic  process  has,  of  late,  been  largely  substituted  ; 
but  the  crib  has  been  used  where  the  amount  of  work  would 
not  justify  the  necessary  first  cost  of  the  pneumatic  plant  or, 
for  the  same  cause,  it  would  be  more  expensive.  Of  late, 
however,  in  a  few  instances  foundation  beds  at  a  greater 
depth  than  100  feet  below  the  surface  have  been  required.  As 
this  depth  is  generally  considered  the  limit  of  the  pneumatic 
process,  builders  have  resorted  again  to  the  use  of  the  crib, 
either  constructed  of  timber  or  iron,  and  to  the  iron  cylinder. 
Three  examples  of  this  method  will  be  briefly  described. 

4.  The  design  of  a  timber  crib  suitable  for  the  above 
described  purposes  is  fully  shown  in  Plate  XI.  Fig.  2  shows  a 
plan  or  horizontal  section  Fig.  1  shows  a  cross-section  and 
part  elevation  ;  Figs.  3  and  4,  Plate  XII,  show  other  details,  etc. 


THE  OPEN  CRIB. 


265 


This  is  a  good  example  of  the  general  construction  of  a  crib, 
although  it  was  in  part  designed  for  a  combined  crib  and 
pneumatic  caisson.  It  will  be  more  fully  explained  farther  on. 

5.  One  of  the  longest  and  largest  structures  in  which  the 
open-crib  method  was  used  in  the  foundations  is  the  Pough¬ 
keepsie  Bridge  across  the  Hudson  River,  New  York,  full  de¬ 
scriptions  and  illustrations  of  which  can  be  found  in  the  Engi¬ 
neering  News  and  the  Engineering  and  Mining  Journal.  The 
following  are  the  principal  points  of  interest :  There  were  two 
cantilever  spans  of  548  ft.,  and  two  counter  balance  or  anchor¬ 
age  arms  of  201  ft.  each,  one  cantilever  span  546  ft.,  and  two 
contiguous  through  trusses  of  525  ft. — giving  a  total  length  be¬ 
tween  end  piers  of  3094  ft.,  and  including  viaduct  approaches 
6767  ft.  The  grade  on  the  approaches  was  66  ft.  per  mile; 
clear  height  of  structure  above  high-water  130  ft.,  making  base 
of  rails,  as  deck  spans  were  used,  212  ft.  above  high-water. 
All  masonry  was  of  first  class  for  facing  stones,  the  backing 
being  of  concrete  with  large  stones  imbedded,  so  as  to  tie  the 
face  and  backing  thoroughly  through  the  entire  pier,  as  has 
been  described  under  the  head  of  masonry.  The  masonry 
rested  on  the  cribs  at  about  10  ft.  below  high-water,  and  was 
built  to  about  30  ft.  above  high-water ;  on  top  of  the  masonry 
steel  towers  about  100  ft.  high  were  erected,  upon  which  the 
superstructure  rested.  To  a  depth  of  100  ft.  or  more  below 
high-water,  the  bed  of  the  river  was  composed  of  silt,  clay,  and 
sand,  underlaid  by  layers  of  a  firm,  coarse  gravel,  between 
which  and  the  rock,  which  was  about  140  ft.  below  high-water, 
there  was  found  a  bed  of  compact  gravel,  upon  which  the 
structure  finally  rested  at  a  depth  of  about  135  ft.  below  low- 
water.  There  were  4  cribs  of  the  same  general  design  and  di¬ 
mensions.  Bottom  dimensions  60  X  100  ft.,  height  104  ft. ;  the 
dimensions  decreased  somewhat  toward  the  top,  giving  a  regu¬ 
lar  batter;  they  were  built  in  the  main  of  12  X  12  in.  hemlock, 
except  the  timbers  which  formed  the  cutting  edges  ;  these  were 
of  white  oak.  The  lower  section  of  the  crib  of  about  20  ft.  in 
height  was  built  of  the  usual  V-shaped  section  of  solid  timbers 
for  the  outside  and  cross-walls,  similar  to  the  lower  part  of  crib 


266  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


shown  in  Plate  XI.  There  was,  however,  only  one  cross-wall. 
The  annexed  diagram,  Fig.  7,  shows  horizontal  section  at  bot¬ 
tom  of  cutting  edge  (see  dotted  lines),  and  also  at  a  point  20  ft. 
above,  as  seen  by  the  full  lines.  C,  C,  C  shows  the  cutting 
edges  of  the  outside  and  middle  walls  ;  B,  B,  B  cross-bulk¬ 
heads  2  ft.  thick  dividing  the  enclosed  space  into  14  cells  or 
pockets,  open  bottom  and  top  and  extending  from  bottom  to 
top  of  crib.  These  are  the  dredging  chambers  or  compartments. 
The  width  of  the  cutting  edges  was  only  a  few  inches,  and 
these  walls  then  increased  in  the  height  of  20  ft.  to  10  ft.  on 
the  sides,  9  ft.  on  the  ends,  and  16  ft.  in  the  middle  walls;  these 
are  shown  by  the  shaded  rectangles.  Upon  these  solid  walls 
the  double  walls  of  the  crib  above  was  built  which  formed  the 
cells  or  pockets  for  the  concrete  filling.  It  is  seen  that  the 
dredging  chambers  D,  D ,  D,  are  for  the  4  end  ones  19  x  30  ft. 
:=  570  sq.  ft.  at  the  plane  of  the  cutting  edge,  and  the  inter¬ 
mediate  ones  are  10  X  30  ft.  =  300  sq.  ft. ;  whereas  at  20  ft. 
above  in  the  plane  of  the  shaded  portions  all  chambers  are 
10  X  12  ft.  =120  sq.  ft.,  and  continue  this  size  to  the  top 
of  the  crib.  Such  cribs  are  built  either  partly  on  shore  and 
then  launched  or  entirely  while  floating  ;  when  a  sufficient 
height  is  built  to  reach  from  the  bed  of  the  river  to  a  point 
somewhat  above  water  surface  they  are  floated  into  position 
and  held  by  anchors,  or  clusters  of  piles,  or  by  cribs  loaded 
with  stone  and  sunk  at  convenient  points.  The  building  of 
the  walls  of  the  crib,  the  weighting  of  the  caisson  with  concrete, 
gravel,  or  broken  stone  is  then  proceeded  with.  The  material 
is  dredged  from  the  bottom  through  the  open  chambers 
D,  D,  D,  and  as  the  material  is  removed  and  frictional 
resistance  decreases,  the  crib  settles  into  the  soil.  In  this 
structure  the  weight  supplied  was  gravel,  and  afterward  this 
gravel  was  removed,  as  I  understand  the  description,  and  then 
these  same  pockets  filled  with  concrete,  as  was  also  the  dredging 
chambers  D.  The  settling  of  the  caisson  was  somewhat  un¬ 
certain  and  irregular,  dropping  sometimes  as  much  as  10  ft.  at 
once.  This  uncertain  and  irregular  settling  is  one  of  the  diffi¬ 
culties  attending  this  open-crib  method.  Under  ordinary  cir- 


Open  Timber  Crib,  Poughkeepsie  Bridge. 


THE  OPEN  CRIB. 


267 


Plan  and  Horizontal  Section  Fls,  7. 


268 


A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


cumstances  with  the  walls  of  the  pockets  well  calked ;  there 
should  be  no  difficulty  in  using  concrete  for  the  weight,  and 
thereby  saving  the  time  and  cost  of  first  filling  them  with 
gravel  or  stone  and  subsequently  removing  the  same.  At 
least  such  is  the  writer’s  experience  in  cribs  40  or  more  feet  in 
height,  even  though  some  of  the  pockets  were  often  left  unfilled 
to  the  depth  of  15  or  20  ft.  below  the  water  surface.  A  suffi¬ 
cient  margin  on  the  height  of  the  crib  should  always  be  pro¬ 
vided  to  keep  its  top  above  water,  and  but  little  pumping 
should  be  necessary  to  keep  the  pockets  free  of  water.  Much 
of  this  concreting  must  have  been  done  under  water,  which 
certainly  is  to  be  avoided  if  practicable.  If  such  pockets  had 
to  be  filled  first  with  gravel  or  broken  stone,  which  is  then 
removed  and  replaced  with  concrete  under  water,  it  would 
have  probably  resulted  in  as  good  a  job,  if  pipes  a  few  inches  in 
diameter  with  a  series  of  holes  at  different  levels  had  been  built 
in  the  gravel  or  broken  stones  at  intervals,  and  instead  of  remov¬ 
ing  the  material,  to  have  poured  a  grout  made  of  cement  alone, 
or  at  most  with  1  cement  and  1  sand,  into  these  pipes,  the  head 
would  force  it  through  the  holes  and  out  between  the  gravel 
or  stone,  thereby  more  or  less  perfectly  filling  all  interstices, 
, and  doubtless  making  as  good  a  concrete  as  that  ordinarily  re¬ 
sulting  from  concreting  under  water.  A  somewhat  similar  plan 
has  been  tried,  not  on  such  an  important  and  extensive  work, 
perhaps,  but  is  said  to  have  given  good  results.  The  above 
described  structure  is  specially  noted  for  the  size  and  height  of 
the  cribs  and  the  depth  of  135  ft.  sunk  below  high-water. 
Although  in  many  details  the  design  and  construction  of 
these  cribs  may  be  different,  yet  the  figures  of  Plate  XI 
considered  as  a  crib  alone,  without  the  shafts,  pipes  and 
horizontal  partitions  or  roofs  of  the  separate  chambers,  will 
represent  a  good  design  of  all  forms  of  open  cribs;  hence  more 
elaborate  drawings  showing  in  details  the  cribs  of  the  Pough¬ 
keepsie  piers  are  omitted,  and  for  these  the  reader  is  referred 
to  the  magazines  mentioned. 

6.  Another  bridge  of  great  length  and  involving  many 
difficulties,  in  which  the  open-crib  method  was  used,  was 


THE  OPEN  CRIB. 


269 


recently  constructed  by  the  Union  Bridge  Co.,  of  New 
York,  and  known  as  the  Hawkesbury  Bridge,  in  New  South 
Wales.  In  this  case  the  cribs  were  constructed  entirely  of 
iron  ;  the  horizontal  sections  of  the  crib  were  rectangular 
with  rounded  ends,  spreading  out  from  a  point  about  twenty 
feet  above  the  bottom.  Except  in  regard  to  the  shape  of 
the  cribs,  the  number  of  dredging  tubes  or  cylinders,  and  the 
thickness  and  the  strength  of  the  plates,  angle-irons,  etc.,  the 
elevation  given  for  the  crib  of  the  Diamond  Shoals  Light¬ 
house,  designed  by  Messrs.  Anderson  &  Barr,  will  be  ample 
without  further  drawings  to  represent  this  particular  case. 
And  as  Plate  XI  has  been  taken  as  a  fair  type  for  the  con¬ 
struction  of  all  timber  cribs,  so  may  the  figures  in  Plate  X  be 
taken  as  a  fair  type  of  the  all-iron  cribs.  Before  giving  some 
of  the  details  of  the  Hawkesbury  Bridge  a  few  remarks  on  the 
general  construction  of  iron  cribs  will  not  be  out  of  place.  By 
referring  to  Plate  X  it  will  be  seen  that  the  lower  section  of 
the  crib  flares  outward  at  a  considerable  angle ;  this  has  doubt¬ 
less  been  characteristic  of  iron  cribs,  whereas  in  Plate  XI  the 
batter  or  outward  flare  is  very  slight,  and  the  same  may  be 
seen  in  the  plates  showing  pneumatic  caissons.  In  either  case 
the  object  is  twofold.  First,  it  increases  the  area  of  the  base, 
thereby  reducing  the  unit  pressure  on  the  foundation-bed  ;  and, 
secondly,  is  supposed  to  facilitate  the  sinking  of  the  caisson  or 
crib  by  reducing  the  friction  on  the  exposed  surfaces.  So  far 
as  the  first  consideration  is  concerned,  the  bottom  could  be 
made  of  the  required  area,  this  continued  for  a  certain  height, 
and  the  area  reduced  abruptly  to  the  size  required  for  the 
structure  above;  this,  then,  has  no  material  importance.  As 
to  the  batter  facilitating  the  sinking  it  has  generally  been 
considered  as  absolutely  necessary  to  have  some  batter ; 
the  amount,  however,  has  been  different  in  different  designs. 
Mr.  Anderson,  who  has  had  great  experience  in  sinking  deep 
cribs  and  cylinders,  expresses  the  opinion  that  in  running 
sand  and  silt  it  makes  but  little  difference  whether  they  have 
any  batter  or  not ;  but  if  the  material  is  tenacious,  as  in  clay 
and  compact  silt,  that  a  vertical  surface  on  the  outside  of  the 


27°  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 

lower  section  is  to  be  preferred,  as  the  material  will  not  other¬ 
wise  close  in  on  the  sides  of  the  caisson,  and  that  it  would  be 
more  difficult  to  guide  and  hold  the  structure  in  a  proper  posi¬ 
tion.  To  confirm  his  view  he  states  that  both  plans  were  tried 
in  the  Hawkesbury  foundations,  and  all  of  the  trouble  occurred 
with  the  inclined  sides,  and  little  or  none  with  those  cribs  that 
had  vertical  sides.  The  following  table  gives  the  depths  sunk 
and  total  heights.  1  he  tops  of  the  piers  were  42  feet  above  low- 
water  ;  difference  between  high  and  low  water,  about  5  feet. 


Depth  from 

Depth 

Total  Height  from 

Low-water  to 
River  Bed. 

Below  River 
Bed. 

Bottom  to  Top 
of  Pier. 

Pier  No.  1 

38  ft. 

55  ft.  8  in. 

135  ft.  8  in. 

“  2 

40  “ 

108  “  1  “ 

190  “  1  “ 

“  3 

43  “ 

96  “  0  “ 

181  “  0  “ 

“  4 

21  “ 

11S  “  6  “ 

181  “  6  “ 

“  5 

I9i  “ 

117  “  5  “ 

178  “  11  “ 

“  6 

47  “ 

108  “  0  “ 

197  “ 

Some  difficulties  weie  encountered,  as  would  have  been  an¬ 
ticipated  in  a  structure  of  such  magnitude. 

T  he  spans  were  constructed  on  false  work  erected  on  very 
large  barges,  and  floated  in  between,  and  then  lowered  on  the 
piers. 

The  length  of  this  bridge  was  2896  ft.  in  length.  The  depth 
to  be  sunk  was  as  shown  in  the  above  table,  through  water, 
mud,  and  sand,  finally  resting  on  a  bed  of  compact  gravel. 
Such  were  the  general  dimensions,  requirements,  and  results. 
In  1884  invitations  were  extended  to  the  bridge  builders  in 
many  parts  of  the  world.  The  builders  were  to  submit  their 
own  plans,  both  for  the  substructure  and  the  superstructure, 
subject  to  certain  limitations  as  to  dimensions  and  strength  of 
materials.  A  large  number  of  plans  were  submitted  by  Eng¬ 
lish,  French,  and  American  builders,  which  resulted  in  the 
contract  being  awarded  to  the  Union  Bridge  Co.,  of  New  York, 
for  the  gross  sum  of  about  $1,835,000.  No  official  or  full  par¬ 
ticulars  of  this  structure  have  been  published  by  the  builders. 
Tht  following  general  facts,  to  which  have  been  added  some 
calculations  of  weights,  resistances,  etc.,  by  the  writer,  are 
taken  from  the  columns  of  the  Engincervig  News.  The  total 


THE  OPEN  CRIB. 


271 


length  was  divided  into  five  spans  416  ft.  long  each,  and  two 
spans  each  408  ft.  long,  by  six  piers  and  two  abutments.  As 
the  depth  to  be  sunk  far  exceeded  the  generally  accepted  limit 
of  the  pneumatic  process,  it  was  determined  to  use  the  open- 
crib  method.  The  crib  was  constructed  entirely  of  iron.  Except 
that  the  enclosing  walls  were  composed  of  iron  plates  stiffened 
by  angle-irons  and  strong  iron  braces  between  the  double  walls, 
the  general  design  was  the  same  as  in  timber  cribs.  The  iron 
plates  of  the  outside  and  partition  walls  were  f  in.  thick,  the 
necessary  weight  to  sink  the  crib  being  deposited  between 
walls.  These  walls  enclosed  three  tubes  or  cylinders  8  ft.  in 
diameter  ;  these  extended  to  about  20  ft.  from  the  bottom,  at 
which  point  they  commenced  to  swell  out  in  a  bell  or  funnel 
shaped  mouth  to  the  bottom  edge,  forming  with  the  outside 
and  partition  walls  strongly  built  and  connected  cutting  edges. 
The  horizontal  sections  were  rectangular  with  rounded  ends, 
the  dimensions  of  the  bottom  section  being  52  X  24  ft.;  these 
dimensions  gradually  decreasing  upward,  so  that  at  a  point 
twenty  feet  from  the  bottom  the  cross-section  was  reduced  to 
48  X  20  ft.,  and  thence  continued  at  these  dimensions  for  a  height 
of  about  155  ft.  to  low- water.  This  was  built  up  in  sections  of 
about  5  ft.  as  the  dredging  and  sinking  progressed.  The  tubes 
were  connected  with  the  side  and  partition  walls  by  strong  iron 
braces.  The  entire  open  space  around  the  tubes  was  filled  with 
concrete  as  the  sinking  progressed  ;  this,  with  the  weight  of 
iron,  overcame  the  resistance.  The  material  was  dredged  out 
through  the  tubes  by  means  of  the  Anderson  Automatic  Dredge  ; 
each  bucketful  had  to  be  lifted  the  full  height  of  the  crib  at  the 
time  and  deposited  in  the  water  or  in  barges.  When  the  proper 
depth  was  reached  the  tubes  were  filled  with  concrete,  de¬ 
posited  under  water.  On  top  of  the  crib  masonry  piers  were 
constructed,  about  40  ft.  high  ;  these  piers  were  42  X  14  ft.  on 
top,  and  46  X  18  ft.  at  the  bottom,  leaving  a  margin  of  about 
1  ft.  all  around  on  top  of  the  crib.  The  piers  seem  to  have  been 
constructed  of  two  circular  columns  of  masonry  14  ft.  in  diam¬ 
eter,  and  28  ft.  centres,  connected  by  a  rectangular  wall  6  ft. 
thick  at  top,  thereby  saving  some  masonry. 


272  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 

7.  The  difficulties  in  this  method  of  sinking  such  large 
structures  are  many.  Great  skill  is  required  in  handling  the 
dredges  so  as  to  excavate  the  material  uniformly  and  close  up 
to  the  sides  of  the  cutting  edges  at  such  great  depths  below 
the  surface  of  the  water ;  the  importance  of  which,  in  sinking 
caissons  and  cribs,  is  very  great,  in  order  to  maintain  the  struct 
ure  in  a  vertical  position  and  prevent  careening  and  conse¬ 
quent  sinking  out  of  line  and  position.  But  the  success  attend¬ 
ing  such  efforts  fully  establishes  its  practicability,  though 
much  is  left  to  blind  chance.  The  sinking  must,  to  a  great 
extent,  take  care  of  itself.  Again,  if  obstacles,  such  as  old 
wrecks,  drift,  logs,  etc.,  are  met  with,  the  removal  of  these 
causes  great  trouble  and  delay  with  its  attending  cost,  as  it  is 
by  no  means  an  easy  job  to  remove  such  obstructions  in  the 
pneumatie  caisson,  where  they  can  be  seen  and  reached. 
Much  of  the  concrete  is  of  necessity  deposited  under  water, 
the  value  of  which  was  fully  discussed  under  the  head  of  con¬ 
crete.  If  deposited  with  care,  the  operation  is  slow  and  ex¬ 
pensive,  and  without  care  it  is  no  better  than  so  much  broken 
stone,  and  perhaps  not  much  better  with  any  degree  of  care. 

Lastly,  the  frictional  resistance  of  the  material  on  the  ex¬ 
terior  surface  of  the  caisson  is  enormous,  especially  if  the  sink¬ 
ing  is  intermittent,  allowing  intervals  of  rest,  during  which  the 
material  closes  in  on  the  caisson.  This  requires  corresponding 
and  enormous  weight  to  overcome  it.  This  resistance  may  be 
many  hundred  pounds  per  square  foot  of  surface.  As  an  illus¬ 
tration,  the  writer  has  made  the  following  calculations  on  this 
structure:  Allowing  the  low  unit  of  resistance  of  250  lbs.  per 
square  foot,  the  total  resistance  must  have  been  12,000  X  250 
=  3,000,000  lbs.  The  weight  of  iron,  roughly  estimated, 
would  be  550,000  lbs.,  leaving  2,450,000  lbs.  of  concrete  to  be 
added,  which,  at  125  lbs.  per  cubic  foot,  would  require  20,000 
cubic  feet,  and  allowing  a  reasonable  excess,  say  1000  cubic 
yards.  Again,  the  estimated  total  weight  on  the  foundation 
bed  would  be  98,806X125  =  12,351,150  lbs.  of  concrete: 
weight  of  iron  550,000  lbs.;  masonry,  15,300  X  160  =  2,448,000 
lbs.;  superstructure  and  load,  2,1 13,280  lbs,  or  a  total  of  17,- 


THE  OPEN  CRIB. 


273 


462,430  lbs.,  or  13,992  lbs.  =  7.0  tons  per  square  foot,  not  con¬ 
sidering  the  frictional  resistance,  or  5.8  tons,  allowing  for  it. 
The  writer  regrets  his  inability  to  give  fuller  information  on 
this  structure. 

8.  In  1885  a  prominent  bridge  builder  consulted  with  the 
writer  in  regard  to  the  cheapest  and  best  method  of  reaching 
such  a  depth,  as  the  pneumatic  process  was  considered  out 
of  the  question,  and  it  was  feared  that  the  open-crib  method 
would  prove  impracticable  on  account  of  the  many  difficulties 
and  objections  already  mentioned.  Being  so  fully  occupied 
at  that  time,  he  could  not  give  the  necessary  consideration  to 
the  matter.  But  in  the  following  year  he  designed  a  structure 
which  was  intended  to  be  a  combination  of  the  open  crib  and 
pneumatic  caisson,  involving  some  new  features  which  were 
subsequently  patented.  This  will  be  more  fully  explained  in 
another  article,  after  explaining  the  pneumatic  process. 

9.  The  third  example  of  the  open-crib  method  will  be 
briefly  alluded  to.  It  was  required  to  construct  a  bridge 
across  a  wide,  deep  bayou  at  Morgan  City,  Louisiana.  The 
material  of  the  bed  of  the  stream  was  very  soft,  with  consider¬ 
able  depth  of  water  over  it.  Several  plans  had  been  discussed 
and  submitted  while  the  writer  was  connected  with  the  road. 
Among  them  was  the  Cushing  cylinder  piers,  and  timber  piles 
with  cast-iron  cylinders  connecting  with  them  at  or  near  the  bed 
of  the  river,  as  well  as  others  of  more  or  less  cost.  But  the 
work  being  abandoned,  nothing  had  been  done  beyond  driving 
a  few  piles.  Subsequently,  on  the  renewal  of  the  work,  it 
was  determined  to  sink  iron  cylinders  by  means  of  dredging 
out  the  material.  These  cylinders  were  eight  feet  in  diameter. 
Below  the  bed  of  the  river  they  were  made  of  cast-iron  in  sec¬ 
tions  10  ft.  long,  with  if-in.  metal  thickness,  strongly  bolted 
together  through  internal  flanges.  Above  the  bed  of  the  river 
wrought-iron  plate,  §•  in.  thick  was  used,  riveted  together  and 
stiffened  with  angle  irons.  The  material  was  dredged  out 
from  the  interior  of  the  cylinder,  and  as  the  cylinder  settled 
sections  were  built  on  top.  By  these  means  they  were  sunk 
a  hundred  or  more  feet  into  the  solid  material.  After  reaching 


274  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 

the  proper  depth,  they  were  filled  with  concrete.  The  stabil¬ 
ity  of  such  small  columns,  having  long  distances  unsupported, 
has  been  repeatedly  noticed.  They  certainly  cannot  be  re¬ 
garded  as  possessing  any  great  excess  of  stability  where  they 
are  subjected  to  heavy  pressures  or  great  shocks,  especially  as 
the  concrete  has  to  be  generally  deposited  under  water.  The 
above  examples  illustrate  the  most  recent  open-crib  and  cyl¬ 
inder  constructions  in  which  depths  have  been  reached  exceed¬ 
ing  that  to  which  the  pneumatic  process  is  generally  consid¬ 
ered  applicable,  which  will  now  be  explained. 


Article  XLVIII. 

THE  PNEUMATIC  CAISSON. 

10.  BEFORE  describing  the  designs  and  construction  of 
caissons,  it  will  be  as  well,  to  avoid  repetition,  to  briefly  con¬ 
sider  certain  general  principles  applicable  in  all  cases,  and  also 
the  design  and  uses  of  certain  parts  common  to  all. 

11.  As  the  name  indicates,  the  air  is  an  essential  element 
to  be  considered,  whether  simply  used  to  sink  the  caisson,  1st, 
where  a  vacuum  is  made  by  exhausting  the  air  from  the  interior 
of  an  air-tight  cylinder  or  box,  and  the  unbalanced  atmos¬ 
pheric  pressure  of  15  lbs.  per  square  inch  of  exposed  surface, 
causing  it  to  sink  into  the  underlying  material.  This  is  called 
the  vacuum  process :  it  may  be  said  that  it  is  rarely,  if  ever, 
used  now  ;  2d,  where  the  air  is  compressed  into  a  cylinder 
or  box,  which  drives  the  water  out,  so  that  the  material  can 
be  excavated  and  removed  from  the  interior,  which  is  called 
the  air  or  working  chamber,  lifting  it  out  in  buckets,  or 
allowing  the  air  to  blow  the  material  out  through  pipes  prop¬ 
erly  regulated  by  valves,  or  forcing  it  out  by  water  pressure. 
This  is  known  as  the  compressed-air  or  pneumatic  process. 
This  latter  term  is  now  commonly  confined  to  the  use  of  com¬ 
pressed  air. 

12.  The  fundamental  principle  underlying  this  is  simply 


THE  PNEUMATIC  CAISSON. 


275 


that  the  atmospheric  pressure  of  15  lbs.  per  square  inch  will 
support  a  column  of  water,  in  a  tube  or  pipe  from  which  the 
air  has  been  exhausted,  of  about  34  ft.  high,  when  the  open 
end  is  immersed  in  a  body  of  water ;  or  1  lb.  will  balance  a 
column  of  27  ins.  high.  Practically  these  heights  cannot  be 
supported,  as  a  perfect  vacuum  is  almost  impossible.  But  it 
is  commonly  stated  that  we  must  have  1  lb.  pressure  for  every 
2\  ft.  of  depth  below  the  water  surface,  to  keep  the  water  out 
of  the  working  chamber.  The  actual  pressure  is  15  lbs.  more, 
as  we  have  to  balance  a  like  pressure  on  the  surface  of  the 
water  outside  the  caisson  ;  this  excess  is  constant  for  all 
depths.  So  that  if  the  depth  below  the  water  surface  is  90  ft. 
the  actual  air  pressure  in  the  caisson  is  about  45  -f-  15  =  60  lbs. 
The  uplifting  effect  is,  however,  only  45  lbs.  Ordinarily,  it 
becomes  necessary  to  reduce  the  air  pressure  in  the  caisson 
very  materially  at  times  in  order  to  allow  the  caisson  to  sink ; 
at  other  times,  however,  it  is  necessary  to  cease  altogether 
adding  weight  to  the  caisson  to  prevent  a  continuous  or  too 
rapid  sinking.  This,  of  course,  depends  both  upon  the  actual 
resistance  at  the  lower  or  cutting  edge  of  the  caisson,  which 
may  or  may  not  be  very  great,  and  upon  the  frictional  re¬ 
sistance  on  the  exterior  surface  of  the  caisson  and  the  struct¬ 
ure  upon  it.  It  is  therefore,  in  general,  better  to  have  as  little 
frictional  resistance  on  the  side  surfaces  as  practicable,  and  to 
provide  as  great  a  direct  resistance  at  or  a  little  above  the 
cutting  edge  as  is  consistent  with  economy  and  convenience 
of  construction  and  subsequent  ease  of  prosecuting  the  work. 

13.  As  the  working  chamber  should  be  practically  air-tight, 
some  special  means  of  entering  and  leaving  the  working  cham¬ 
ber  must  be  provided.  The  air-lock  has  this  object  in  view, 
and  wherever  it  is  placed  or  whatever  its  design,  it  must  be  an 
air-tight  box  with  two  doors,  both  opened  toward  the  greatest 
pressure — that  is,  toward  the  air-chamber  or  some  air-tight 
channel  or  shaft  communicating  with  it.  These  doors  open 
inward  or  downward,  and  when  shut  must  bear  against  rub¬ 
ber  gaskets,  so  as  to  practically  exclude  the  passage  of  air ;  as 
it  is  the  air-pressure  itself  that  keeps  the  door  shut,  one  of 


2j6  a  practical  treatise  on  foundations. 

them  will  always  be  open.  Strong  and  tight  iron  shafts  are 
built  into  the  caisson,  and  should  always  reach  well  above  the 
surface  of  the  water;  the  main  shaft  through  which  the  men 
enter  and  leave  need  not  be  over  4  ft.  in  diameter.  This  is 
made  in  sections,  which  are  bolted  together  through  internal 
flanges,  between  which  rubber  bands  or  some  soft  and  imper¬ 
vious  substance  is  placed,  so  as  to  render  the  joint  air-tight. 
Ordinarily  red  lead  worked  up  with  short  strands  of  ordinary 
lampwick  will  answer  every  purpose,  it  is  easily  obtained  and 
applied.  A  section  of  the  shaft  itself  can  be  converted  into  an 
air-lock  by  connecting  two  doors  to  it,  or  a  specially  designed 
air-lock  can  be  connected  with  the  shaft  at  its  top,  bottom,  or 
any  intermediate  point.  The  writer  prefers  the  air-lock  at  the 
top,  and  that  it  shall  also  be  simply  a  section  of  the  shaft ;  as 
any  section  can  be  converted  into  an  air-lock,  or  the  whole 
shaft  if  so  desired.  This  arrangement  possesses  many  con¬ 
veniences,  and  is  much  safer  than  when  located  at  or  near  the 
bottom.  It  frequently  happens  that  men  are  driven  suddenly 
from  the  working  chamber,  and  if  the  lock  is  at  the  top  they 
can  all  climb  up  the  shaft  and  be  in  safety,  while  the  air  is 
being  equalized  so  that  the  lower  door  of  the  air-lock  can  be 
opened,  or  if  open  they  can  enter  the  air-lock  without  delay  or 
confusion,  or  the  danger  of  some  one  closing  the  door  upon 
them.  On  the  contrary,  with  the  air-lock  at  or  near  the  bottom, 
the  men  have  no  place  to  enter  and  be  safe  if  the  lower  door 
of  the  lock  is  closed  ;  a  few  minutes’  delay  may  be  fatal  to  many, 
or  they  all  may  not  be  able  to  enter  the  air-lock  in  the  con¬ 
fusion  and  often  cowardice  shown  by  some  men  in  the  face  of 
danger.  The  air-lock  being  a  part  of  the  shaft  is  a  mere  mat¬ 
ter  of  convenience. 

14.  A  smaller  shaft,  not  over  18  ins.  or  2  ft.  diameter,  for 
letting  concrete  or  other  material  into  the  working  chamber,  is 
also  used.  It  is  better  to  have  at  least  two  of  these  ;  they  are 
provided  with  a  door  at  top  and  bottom  only,  the  entire  shaft 
being  an  air-lock.  In  addition  to  these,  pipes  from  4  to  6  ins. 
diameter  are  also  built  into  the  caisson— the  larger  diameter  for 
connection  with  the  air-hose  and  force  pump  for  water,  the 


THE  PNEUMATIC  CAISSON. 


2  77 

smaller  diameter  for  use  in  blowing  out  the  material.  There 
should  be  a  number  of  these  distributed  around  the  caissons. 
All  pipes  should  be  provided  with  the  best  valves,  and  when 
not  in  use  should  be  capped  with  a  cap  screwed  on  to  the  pipe 
above  the  surface  and  stopped  by  plugs  below  to  prevent  any 
possible  chance  of  a  sudden  escape  of  the  compressed  air. 

15.  The  use  of  the  air-lock  can  now  be  easily  understood. 
Compressed  air  is  rarely,  if  ever,  required  until  the  caisson 
rests  firmly  on  the  bed  of  the  river  in  its  proper  position  for 
the  pier.  As  soon  as  it  does  so  rest,  the  doors  being  both  open, 
air  connections  are  made  between  the  proper  pipe  and  the  air 
compressors ;  all  other  pipes  or  avenues  through  which  the  air 
could  escape  being  closed,  the  lower  door  is  lifted  by  a  small 
tackle  against  its  bearing,  and  the  compressors  are  then  started. 
It  requires  only  a  few  pounds  of  pressure  to  hold  the  door  in  posi¬ 
tion.  When  the  pressure  gauge  indicates  a  pressure  required  for 
the  then  depth,  men  enter  the  air-lock  through  the  other  door¬ 
way,  its  door  swinging  freely.  This  door  is  then  lifted  into  posi¬ 
tion  by  the  lock  tender  on  the  outside ;  the  valve  in  the  upper 
door  or  in  any  other  position  in  which  it  may  be  placed  is 
closed,  and  the  valve  in  the  lower  door  or  opening  into  the 
main  shaft  at  some  point  below  the  air-lock  is  opened.  The 
compressed  air  rushes  into  the  air-lock,  and  continues  to  do  so 
at  a  lessening  velocity  until  the  air  in  the  lock  is  at  the  same 
pressure  as  that  in  the  working  chamber;  it  is  then  said  to  be 
equalized.  The  lower  door  would  now  open  of  its  own  weight,  if 
it  were  not  held  in  position  by  a  tackle  in  the  air-lock.  As  the 
pressure  on  both  sides  is  now  the  same,  the  lock  tender  on  the 
inside  allows  the  door  to  open,  and  the  men  descend  by 
means  of  an  iron  ladder  fastened  to  the  sides  of  the  shaft  into 
the  working  chamber.  A  thorough  examination  is  made  to  see 
that  there  are  no  leaks  ;  complete  the  interior  bracing  if  not 
already  completed,  and  see  in  short  that  everything  in  the  in¬ 
terior  is  all  right.  To  get  out  they  ascend  the  shaft,  enter  the 
lock  through  the  lower  or  open  door-way,  lift  this  door  to  its 
place,  close  the  lower  valve,  and  open  the  upper  valve,  which  al¬ 
lows  the  compressed  air  in  the  lock  to  escape  into  the  open  air. 


2 7s  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 

In  a  short  tune  this  pressure  will  be  reduced  to  that  of  the  at¬ 
mosphere,  the  upper  door  is  lowered  by  the  outside  lock  ten¬ 
der,  and  the  men  pass  out.  I  he  above  operations  have  to  be 
repeated  each  time  that  a  man  passes  in  or  out  of  the  caisson. 

16.  If  everything  is  ready  below,  a  gang  or  shift  of  men 
now  passes  into  the  lock  and  thence  into  the  caisson,  and  the 
work  of  excavating  the  material  in  the  caisson  is  commenced.. 
So  long  as  the  depth  is  not  over  from  60  to  70  ft.  below  the 
water  suiface,  only  three  gangs  or  shifts  are  required  during 
the  24  hours ;  each  shift  working  8  hours  and  resting  16  hours,, 
coming  out  to  lunch  at  about  the  middle  period  of  their  work¬ 
ing  time.  This  will  consume  from  4  to  f  hours,  so  that  they 
only  remain  about  3J  hours  in  the  caisson  at  a  time.  For 
gi eater  depths  the  men  are  divided  into  4  shifts,  working  6 
hours  each,  with  the  same  interval  of  rest  during  this  time,  or 
actually  remaining  in  the  caisson  about  2\  hours  at  a  time.  A 
full  shift  consists  of  1  foreman  and  10  to  20  men,  according  to 
the  size  of  the  caisson,  and  one  outside  and  one  inside  lock 
tender,  this  not  including  the  machinery  men,  such  as  engineers, 
firemen,  pipe-fitters,  etc.,  and  one  or  two  handy  men,  and  over 
all  a  general  superintendent.  The  general  duties  of  these  men 
and  the  mode  of  procedure  will  be  explained  later. 

17-  One  thing  can  be  relied  on  :  so  long  as  the  air  pressure 
required  by  the  depth  is  maintained,  the  water  will  not  rise 
above  the  extreme  lowest  line  of  the  cutting  edge  of  the  cais¬ 
son,  and  in  sinking  through  some  materials  water  has  to  be 
pumped  into  the  caisson  in  order  to  carry  on  the  work.  The 
caisson  must  be  heavily  weighted  before  the  air  pressure  is  put 
on,  or  a  dangerous  tendency  to  lift  and  careen  will  exist.  The 
end  of  the  air  pipe  in  the  caisson  should  be  fitted  with  an 
automatic  valve,  opening  into  the  caisson,  so  that  should  the 
compressors  stop  from  any  cause,  the  air  pressure  will  close  the 
valve  and  prevent  the  escape  of  the  air;  a  simple  circular  plate 
of  iron  with  a  rubber  gasket  sliding  freely  on  two  small  iron 
rods  attached  to  the  end  of  the  pipe,  and  allowing  a  play  of  1^- 
to  2  ins.,  answers  well  the  purpose,  as  it  does  not  prevent  an 
easy  flow  of  air  into  the  caisson,  but  closes  instantly  on  the  air 


THE  PNEUMATIC  CAISSON. 


27  9 


compressors  stopping.  A  small  plunger  pump  connected  with 
the  compressors  forces  a  certain  amount  of  water  in  with  the 
air  to  prevent  its  getting  too  dry  and  hot;  this  is  all  important. 
At  a  depth  of  80  or  90  ft.  the  usual  temperature  in  the  work¬ 
ing  chamber  will  be  from  85  to  90°  Fahr.  This  is  due  to  com¬ 
pressing  the  air.  The  temperature  of  the  air  in  the  air-lock 
will  rise  to  106  to  1 2 5 0  Fahr.,  the  temperature  in  the  air  cham¬ 
ber  being  reduced  by  the  moisture  and  the  cooler  surfaces  on 
the  interior. 

18.  As  to  the  effect  on  men  working  in  compressed  air,  a 
few  remarks  may  be  interesting  and  instructive. 

While  in  the  air-lock  everybody  is  more  or  less  affected 
with  pains  in  the  ears,  known  as  “  blocking.”  With  some  it  is 
intense,  and  many  have  to  reverse  the  valves  and  get  out  before 
the  pressure  is  equalized,  but  the  act  of  swallowing,  blowing 
the  nose,  or  closing  the  nose  and  mouth  and  exhaling  the  air 
from  the  lungs  will  give  ready  relief.  This  trouble  may  arise 
either  on  entering  or  leaving  the  lock.  Again,  in  about  15  or 
20  minutes  after  coming  out  of  the  caisson  many  men  are  at¬ 
tacked  with  severe  pains  in  the  limbs  ;  these  may  be  more  or 
less  intense  and  may  last  a  day,  a  week,  and  sometimes  longer, 
but  seem  to  leave  no  permanent  effects.  These  pains  are  known 
as  the  “  bends.”  Returning  into  the  compressed  air  gives  instant 
relief,  but  they  will  probably  return  on  again  leaving  ;  this 
trouble  is  common,  but  very  many  escape  entirely. 

A  more  serious  trouble  sometimes  happens,  resulting  in  a 
paralysis  of  some  part  of  the  body.  This  will  in  general  be  of 
a  temporary  nature,  but  is  sometimes  lingering  and  often 
permanent ;  but  a  small  per  cent  of  men  will  be  thus  attacked. 
And  lastly,  some  severe  cases  of  paralysis  occur,  from  which 
the  men  die  within  a  few  hours  or  in  a  day  or  two.  Occa¬ 
sionally  a  blood  vessel  in  the  nose  or  ear  will  be  broken, 
some  men  losing  their  hearing  from  this  cause  ;  on  the  con¬ 
trary,  for  some  forms  of  deafness  it  has  been  claimed  that 
exposure  to  compressed  air  affords  more  or  less  relief.  Many 
opinions  and  theories  have  been  advanced  as  to  the  prin¬ 
cipal  causes  of  these  troubles ;  but  most,  if  not  all,  are  unsatis- 


28o  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 

factory,  if  not  entirely  erroneous.  The  writer  had  about  five 
years  of  almost  continuous  experience  in  works  of  this  kind, 
going  almost  daily  into  the  caissons  and  remaining  often  in 
the  pressure  for  hours  at  a  time ;  and  though  not  conscious  of 
any  harmful  effects  of  either  a  temporary  or  permanent  charac¬ 
ter  so  far  as  he  was  concerned,  he  made  a  careful  study  of  the 
effects  on  others,  and  believes  that  much  of  the  trouble  is  due 
to  carelessness  and  indifference  on  the  part  both  of  the  men 
and  managers,  and  even  under  these  circumstances  he  believes 
that  perfectly  healthy  men  have  but  little  cause  of  uneasiness. 

19.  In  going  through  the  air-lock,  as  has  been  stated,  the 
temperature  rises  in  a  few  minutes  from  that  of  the  atmosphere 
at  the  time,  whether  during  freezing  or  milder  weather,  to  at 
least  1060  Fahr.,  causing  a  profuse  perspiration  to  set  up  in  a 
few  minutes.  This  continues  while  below.  On  passing  out 
through  the  air-lock,  as  the  pressure  rapidly  falls,  so  does  the 
temperature  ;  the  perspiration  is  suddenly  checked,  and  a  cold, 
clammy  sensation  follows.  The  men,  with  little  or  no  clothing 
on,  pass  out  into  a  temperature  very  much  lower,  often  well 
below  the  freezing  point ;  they  sit  in  exposed  positions  around 
the  engine  room  or  elsewhere  for  a  half  hour  or  more,  and 
again  go  through  the  same  ordeal.  Entirely  inadequate  ar¬ 
rangements  for  their  protection  or  comfort  are  sometimes  pro¬ 
vided.  Entering  and  leaving  the  caisson  often  happens  many 
times  in  a  period  of  six  or  eight  hours.  While  working  below, 
even  if  the  working  chamber  is  lighted  by  electricity,  which  is 
not  always  or  even  generally  the  case,  it  is  necessary  to  use  can¬ 
dles  to  a  great  extent ;  these  are  especially  prepared,  and  would 
burn  but  slowly  under  ordinary  conditions,  yet  burn  freely  in 
the  compressed  air,  saturating  the  air  with  large  quantities  of 
soot,  which  the  men  breathe  freely  and  constantly,  getting 
their  system  and  lungs  filled  with  it,  and  expectorating  contin¬ 
ually  a  black  mass  from  their  lungs — this  continuing  for  weeks 
even  after  completing  the  work.  The  above  conditions  are 
doubtless  the  most  potent  factors  in  causing  the  caisson  dis¬ 
eases.  It  is  commonly  believed  that  the  actual  pressure  is  the 
cause.  There  is  absolutely  no  evidence  to  sustain  this  opinion, 


THE  PNEUMATIC  CAISSON. 


28l 


beyond  the  blocking  of  the  ears,  which  is  evidently  caused  by 
an  almost  infinitely  small  period  in  time  of  an  unbalanced 
pressure  on  the  outside  or  inside  of  the  drum  of  the  ear,  as 
in  all  other  respects  the  condition  of  the  physical  man  is  per¬ 
fectly  normal,  no  matter  how  long  he  may  remain  under  the 
pressure.  There  is  no  observable  compression  or  subsequent 
puffing  of  the  flesh,  no  restraint  or  other  change  in  his  move¬ 
ments,  or  in  the  use  of  himself,  except  that  he  will  work  and 
hit  harder  and  feel  more  or  less  exhilaration,  which  is  no  doubt 
due  to  an  increase  in  the  supply  of  oxygen,  which  even  over¬ 
comes  the  lassitude  that  would  otherwise  be  caused  by  such  a 
profuse  and  continued  perspiration. 

20.  If  the  writer  is  right  in  his  views,  the  remedy  or  cer¬ 
tainly  an  amelioration  of  the  troubles  is  simple  and  not  ex¬ 
pensive. 

1.  Select  only  healthy  men  for  this  work.  Little  or  no 
attention  is  given  to  this.  The  only  rule  is  to  get  men  and 
get  them  as  cheaply  as  possible.  From  20  to  25  cents  an  hour 
for  eight  hours’  work  is  the  usual  price  paid.  Men  will  do 
from  one  and  a  half  to  two  times  the  work  in  the  caisson  that 
they  will  do  outside. 

2.  Prevent,  at  least,  to  some  extent,  the  sudden  alterations 
of  temperature  through  70  to  90°  Fahr.  day  or  night,  and  in 
all  kinds  of  weather.  A  common  reply  to  this  is  that  the  men 
who  regulate  the  valves  are  instructed  to  pass  the  men  through 
slowly,  and  that  the  valves  are  worked  entirely  by  the  men  in 
the  locks,  \vho  will  be  the  sufferers ;  but  yet  valves  of  compar¬ 
atively  large  apertures  are  given  them.  If  they  were  smaller,  or 
if  not  fully  opened,  the  time  would  be  much  longer  in  passing 
in  and  out,  during  which  time  all  work  must  be  suspended  in 
part  or  entirely ;  this  means  loss  of  time  and  money,  which  is 
not  necessary.  Provide  then  a  lock  or  chamber  connected 
with  the  main  lock,  in  which  men  can  enter  without  obstruct¬ 
ing  the  main  lock,  which  can  be  maintained  at  a  bearable  tem¬ 
perature,  while  the  air  is  being  equalized  ;  let  the  men  wash 
and  dress  themselves,  and  come  out  in  some  sort  of  comfort. 
Any  man  will  get  out  of  a  temperature  of  106°  to  1250  as  fast  as 


282 


A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


possible ;  nor  will  he  remain  in  a  cold,  clammy  condition 
longer  than  possible.  Although  it  may  not  be  practicable  to 
do  away  entirely  with  candles,  the  use  of  them  can  be  materi¬ 
ally  lessened.  These  remedies  will  be  attended  with  some  ex¬ 
pense,  but  they  will  greatly  add  to  the  health  and  useful¬ 
ness  of  the  men,  and  doubtless  enable  us  to  reach  much 
greater  depths  than  too  ft.  by  the  pneumatic  process  with 
vastly  less  danger  and  suffering  than  now  exists  at  depths 
under  100  ft.  below  the  water  surface. 

21.  A  code  of  signals  is  always  used,  by  which  the  men  in 
the  caisson  can  communicate  their  wants  to  those  above.  The 
method  of  simply  knocking  with  an  iron  bolt  on  the  iron  shaft 
or  pipes  is  as  satisfactory  as  any  that  could  be  devised  ;  it  gives 
a  clear,  ringing,  unmistakable  sound,  i  knock  for  more  air, 
2  for  less,  3  for  starting  the  water-pump,  4  that  the  men  are 
coming  out,  etc.,  varied  as  may  be  desired,  answers  all  prac¬ 
ticable  purposes.  The  outside  lock  tender  above  all  should 
be  a  faithful,  wakeful,  and  reliable  man,  ever  on  the  alert 
for  signals  from  below,  as  all  wants  should  be  supplied  immedi¬ 
ately. 

22.  The  immediate  effect  of  reducing  the  air  pressure  even 
by  only  a  few  pounds  is  to  set  up  a  dense  fog.  All  oscilla¬ 
tions  in  the  pressure  should  therefore  be  avoided  as  far  as 
practicable,  and  this  together  with  the  greater  tax  upon  the 
capacity  of  the  compressors  is  the  main  objection  to  forcing 
out  the  material  through  the  pipes  by  means  of  the  compressed 
air ;  a  method  which  in  other  respects  is  more  rapid,  and  in 
many  cases  more  economical  and  satisfactory,  than  any  other 
of  removing  the  material  from  the  working  chamber.  A  4-in. 
pipe  will  easily  carry  gravel,  sand,  mud,  and  bowlders  up  to 
31  ins.  diameter.  It  requires  careful  regulation  or  feeding, 
however,  to  avoid  choking  the  pipes,  and  requires  a  considera¬ 
ble  quantity  of  surplus  air.  For  these  reasons  a  sand  or  mud 
pump  is  often  or  commonly  used. 

23.  A  few  remarks  on  the  necessary  machinery  will  be  useful. 
Several  boilers  of  large  steam-producing  capacity  are  essen¬ 
tial ;  much  time  and  money  are  lost  and  great  inconvenience 


CONSTRUCTION  OF  PNEUMATIC  CAISSONS.  283 


caused  by  the  want  of  them.  The  compressors  have  to  be  run 
continuously  day  and  night,  and  often  in  addition  large  force 
pumps,  electrical  machinery,  and  pumps  for  keeping  water  out 
of  the  cribs  while  concreting  and  out  of  the  coffer-dams  while 
building  the  masonry.  And  after  making  a  liberal  allowance 
for  these  purposes,  at  least  one  extra  boiler  should  be  provided, 
as  some  wear  out,  some  need  repairs,  and  a  largely  increased 
supply  of  steam  is  sometimes  required.  One  good-sized  double 
compressor  will  generally  supply  the  requisite  amount  of  air; 
another  should  always  be  in  reserve.  At  least  one  large 
double  force  pump  should  be  provided.  Other  engines, 
pumps,  etc.,  of  smaller  power  will  be  required.  A  large 
supply  of  pipes,  hose,  machinist  tools,  etc.,  should  be  provided, 
and  with  them  a  first-class  machinist,  as  a  large  amount  of 
fitting,  repairing,  etc.,  must  be  done  on  the  work  and  promptly, 
whether  required  by  day  or  by  night.  This  machinery  is 
generally  mounted  on  one  or  more  barges  and  tied  to  the 
structure.  All  connections  between  the  machinery  and  the 
pipes,  etc.,  should  be  made  by  the  best  make  of  hose,  to  avoid 
any  possibility  of  breaking,  bending,  or  otherwise  deranging 
any  of  the  pipes.  As  a  sudden  escape  of  air  may  cause  not 
only  loss  of  life,  but  serious  damage  to  the  structure.  No  worn- 
out,  broken-down  machinery  or  fittings  of  any  kind  should  be 
allowed. 


Article  XLIX. 

CONSTRUCTION  OF  PNEUMATIC  CAISSONS. 

24.  The  general  design  of  caissons  is  the  same  whether 
made  of  wood  or  iron,  and  consists  of  three  parts,  as  follows : 
1st,  the  walls  of  the  working  chamber ;  2d,  the  deck  or  roof  of 
the  caisson,  with  its  necessary  shafts,  pipes,  etc.,  built  into  and 
through  it  ;  and  3d,  the  necessary  trusses,  braces,  etc.,  to 
strengthen  and  stiffen  the  walls  and  the  roof. 

A  short  description  of  the  design  and  construction  of  some 
of  the  typical  timber  and  iron  caissons  heretofore  used  will 
now  be  given. 


284  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


25.  The  caissons  for  the  foundations  of  the  New  York  and 
Brooklyn  suspension  bridge  are  about  the  largest  timber  cais¬ 
sons  constructed  in  this  country.  They  were  rectangular  in 
cross-section.  The  bottom  dimensions  172  X  102  ft.,  and  at 
top  165  X  95  31^  ft.  high;  thickness  of  the  roof  22  ft.,  and 

were  sunk  78  ft.  below  mean  high-tide,  d  he  frictional  re¬ 
sistance  on  the  sides  varied  from  280  to  600  lbs.  per  square  foot. 
Estimated  pressure  on  a  foundation-bed  of  sand,  7^  tons  per 
square  foot.  The  design  of  the  caisson  was  simple.  All  of 
the  timbers  composing  it  were  12x12  ins.  in  cross-section  and 
laid  horizontally  and  well  bolted  together.  The  height  of  the 
working  chamber  was  9$  ft.;  the  thickness  of  the  walls  varied 
from  6  ins.  at  the  cutting  edge  to  9  ft.  where  it  was  joined  to 
the  roof.  This  was  built  solid  of  timbers  laid  in  courses  one 
on  top  of  the  other,  crossing  each  other  and  bolted  together. 
The  inner  slope  of  these  walls  were  1  to  1,  the  vertical  sec¬ 
tion  being  V-shaped.  These  were  connected  by  cross-wall, 
also  built  solid,  dividing  the  working  chamber  into  compart¬ 
ments  communicating  by  openings  in  the  cross-walls.  On 
these  walls  a  solid  roof  of  square  timbers  in  courses  crossing 
each  other,  22  ft.  thick,  was  constructed,  thoroughly  bolted 
together.  A  cast-iron  shoe  was  placed  on  the  cutting  edge, 
and  under  this  plate-iron  was  bent  extending  up  both  the 
outer  and  inner  slopes  of  the  wall,  and  in  one  of  the  caissons 
the  entire  inner  surfaces  of  the  chamber  was  lined  with  plate- 
iron,  and  also  between  the  fourth  and  fifth  courses  of  the  roof 
a  layer  of  tin  was  placed  and  bent  downward  on  the  outside, 
reaching  to  the  iron  plate  above  mentioned.  These  metal 
linings  were  used  to  prevent  damage  from  fire,  and  also  to 
insure  air-tightness.  The  deck  timbers  were  not  placed  in 
close  contact,  the  intervals  being  filled  with  concrete  or  mor¬ 
tar.  The  usual  pipes,  shafts,  etc.,  were  built  through  the  roof, 
and  in  addition  a  large  shaft  8  ft.  in  diameter,  open  at  both 
ends,  the  lower  end  reaching  into  an  excavation  at  the 
bottom  filled  with  water,  the  water  extending  up  the  shaft. 
This  was  used  for  removing  large  bowlders,  etc.,  by  means  of 
specially  designed  hooks  or  buckets  worked  from  above.  This 
v  as  about  the  only  novel  or  unusual  feature  in  the  design. 


CONSTRUCTION  OF  PNEUMATIC  CAISSONS.  285 


One  of  the  caissons  caught  fire,  which,  being  supported  by  a 
large  quantity  of  oxygen,  burnt  its  way  to  a  considerable  dis¬ 
tance  into  the  roof.  The  caisson  had  to  be  flooded  to  extin¬ 
guish  the  fire.  It  is  not  an  unusual  habit  of  caisson  men  to 
use  the  flame  of  a  candle  to  detect  leaks  in  the  caisson.  There 
is  always  some  danger  in  the  presence  of  so  much  oxygen  and 
combustible  material  of  starting  a  fire.  Other  methods  of 
determining  air-leaks  should  be  used. 

26.  The  caissons  for  the  St.  Louis  bridge,  though  com¬ 
monly  called  iron  caissons,  were  largely  constructed  of  timber 
and  iron  combined.  The  walls  of  the  working  chamber  were 
composed  of  iron  plates,  stiffened  by  angles  and  brackets  ; 
timber  also  being  fastened  to  the  walls,  giving  stiffness  and 
also  affording  an  increased  bearing  surface.  The  decks  of 
these  caissons  were  formed  by  deep  and  strong  girders  or 
beams,  resting  on  the  outside,  and  cross-walls  of  the  air  cham¬ 
ber,  to  the  under  side  of  which  plate-iron  was  riveted  or 
bolted,  forming  a  strong  and  air-tight  roof.  The  space  be¬ 
tween  the  girders  was  filled  with  concrete  or  masonry,  and 
the  regular  masonry  for  the  piers  was  then  built  on  top  of 
this.  As  the  sinking  progressed,  a  timber  coffer-dam,  sheathed 
on  the  outside  with  plate-iron,  was  built  up,  in  which  the  ma¬ 
sonry  was  constructed.  In  the  Brooklyn  bridge  no  coffer-dam 
was  used  ;  the  masonry  commenced  on  the  deck  of  the  cais¬ 
son,  and  was  built  up  as  the  caisson  settled,  so  as  to  keep  its 
upper  surface  above  the  water-line.  In  the  St.  Louis  bridge 
large  open  shafts  were  built  in  the  masonry;  these  were  lined 
with  brick  and  timber,  so  as  to  make  it  water-tight.  The  air¬ 
lock  was  placed  at  the  bottom  of  the  shaft.  The  writer  has 
heard  it  stated  that  in  this,  as  in  some  other  cases,  the  en¬ 
gineers  placed  the  air-locks  at  the  bottom,  leaving  long  open 
shafts,  reaching  above  the  surface  of  the  water,  so  that  the 
men  might  ascend  the  ladders  or  the  shafts  in  the  ordinary 
air.  Whether  this  is  true  or  not,  he  does  not  think  that  cais¬ 
son  men  would  hesitate  to  prefer  to  make  the  ascent  in  com¬ 
pressed  air,  as  there  is  always  a  feeling  of  lassitude  and  an  indis¬ 
position  to  exerting  one’s  self  immediately  after  coming  out  of 
compressed  air,  to  say  nothing  of  the  feeling  of  safety  when  tlm 


286  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 

air-lock  is  at  the  top  of  the  shaft.  The  horizontal  cross-sections  of 
the  St.  Louis  caissons  were  hexagonal  in  shape  to  conform  approx¬ 
imately  to  the  shape  of  the  masonry  piers ;  their  dimensions  at 
the  bottom  were  83  X  70  ft.,  and  64  X  48  ft.  at  a  point  14  ft. 
above.  A  section  of  this  kind  is  easily  built  in  iron,  but  for  tim¬ 
ber  caissons  it  would  present  some  objectionable  features.  These 
caissons,  after  sinking  through  water  and  sand,  finally  rested  on 
rock  at  a  depth  of  109^  ft.  below  the  water  surface.  The  sand 
was  removed  during  the  sinking  by  the  sand  pump,  the  princi¬ 
ple  of  which  is  the  same  as  the  ordinary  injector,  and  will  be 
explained  under  another  example  of  caissons.  The  working 
chamber  of  one  of  the  caissons  was  filled  entirely  with  con¬ 
crete.  But,  as  a  matter  of  economy  in  the  other,  a  wall  of 
concrete  was  built  entirely  around  the  working  chamber,  and 
the  interior  space  was  filled  with  sand.  The  estimated  press¬ 
ure  on  the  foundation-bed  was  19  tons  per  square  foot.  At 
the  time  of  constructing  this  bridge  the  caissons  were  the  larg¬ 
est  ever  used,  and  the  depth  below  the  water  surface  the  great¬ 
est  ever  reached.  All  things  considered,  this  bridge  is  one  of 
the  greatest  structures  in  the  country. 

27.  The  latest,  and  perhaps  the  largest,  structure  of  the 
kind  has  recently  been  completed  across  the  Mississippi  River 
at  Memphis,  Tennessee.  This  bridge  was  opened  for  traffic 
May  12,  1892. 

Although  no  full  and  official  publication  has  been  made  in 
regard  to  this  structure,  the  following  data  and  description 
have  been  obtained  from  reliable  sources. 

The  total  length  of  the  structure  is  7997  ft.,  divided  as 
follows : 


Iron  viaduct  approach  to  bridge  proper .  2300.00  ft. 

Timber  trestle  “  “  “  “  .  3100.00  “ 

Bridge  proper,  divided  into  5  spans  by  6  piers.  The  length  of  the 

spans  were  as  follows  :  1  span  225.83  = .  225.83  “ 

1  cantivever  span.  Cantilever  arms  each  169.38  ft.;  suspended 

truss,  451.66  ft.  Total  length .  790.42  “ 

1  span  cantilever  arm  169.38  ft.,  and  truss  451.66  — .  621.04  “ 

1  through  truss .  621.06  “ 

1  deck  “ .  338.75  “ 

Total  length .  7997.10  ft. 


CONSTRUCTION  OF  PNEUMATIC  CAISSONS.  287 


The  masonry  piers  varied  in  height  from  93  to  158  ft.,  con¬ 
structed  of  Georgia  granite-face  stones  and  Indiana  limestone 
backing. 

There  were  five  pneumatic  caissons,  varying  in  horizontal 
dimensions  from  40  X  22  ft.  to  92  X  47  ft.,  and  in  height  from 
40  to  80  ft.  from  the  bottom  of  caisson  to  bottom  of  masonry,* 
and  sunk  to  depths  from  78  to  1 3 1  ft.  below  high-water  As  it 
was  apprehended  that  a  scouring  action  might  be  caused  by  the 
obstruction  to  the  current  when  the  caisson  was  lowered  to  the 
bed  of  the  river,  rendering  it  difficult  to  properly  level  and 
locate  the  caisson  in  the  commencement  of  the  work,  large 
willow  mattresses,  laced  with  wire,  were  constructed  and  sunk 
to  the  bed  of  the  river  over  the  site  of  the  caisson  by  sufficient 
weight  of  rock.  These  mattresses  were  240  x  400  ft.  square, 
this  affording  a  large,  protected  surface  on  the  bed  of  the  river. 
Upon  these  the  caissons  were  lowered  ;  and  when  they  rested 
firmly  and  the  air-pressure  put  on,  men  descended  into  the 
working  chamber  and  cut  through  the  mattress  along  the  cut- 
ting-edge  of  the  caisson,  allowing  the  caisson  to  sink  through 
the  mattress.  Total  distance  from  top  of  masonry  to  founda¬ 
tion-bed,  which  was  composed  of  clay,  was  about  199  ft.,  and 
below  high-water  about  13 1  ft.,  and  96  ft.  below  low-water. 
The  greatest  immersion  was  108  ft. 

The  anchor  pier  on  land  was  founded  about  50  ft.  below 
the  surface,  and  weighed  2500  tons.  Long  iron  rods  passed 
through  the  masonry,  and  were  fastened  to  a  network  of  iron  I- 
beams  under  the  masonry.  This  made  the  entire  mass  of  the 
pier  act  as  a  unit  in  balancing  the  moving  load  on  the  canti¬ 
lever.  The  cost  of  this  structure  was  about  $3,000,000, 
and  required  about  three  years  in  its  construction.  The  under 
side  of  the  trusses  were  109  ft.  above  low-water  and  75  ft. 
above  high-water.  Height  of  trusses,  about  78  ft.,  and  width 
between  trusses,  30  ft.  between  pin  centres.  The  notable 
features  of  this  structure  were  the  great  length  of  spans  used, 

*  The  designers  of  these  structures  seem  to  make  no  distinction  between 
the  caisson  proper  and  the  crib  above  it;  calling  the  entire  structure  a  caisson. 
The  writer  calls  that  part  a  caisson  shown  in  Fig.  3,  Plate  XVIII.  A  crib 
above  may  or  may  not  be  used. 


288  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 

especially  in  the  cantilevers,  and  the  use  of  large  mattresses  to 
prevent  scouring  action  in  the  early  stages  of  the  work.  Total 
weight  of  superstructure,  19,541,700  lbs.,  or  9771  tons. 

The  above  are  examples  of  the  largest  structures  now  in  ex¬ 
istence  in  which  pneumatic  caissons  of  wood  alone  or  wood  and 
iron  combined  were  used  in  the  construction  of  the  foundations. 

28.  A  description  of  an  all-iron  caisson  will  be  given,  ac¬ 
companied  with  a  skeleton  sketch  of  the  caisson  itself— inter¬ 
esting  and  instructive  not  only  as  a  typical  non  caisson,  but 
also  from  the  special  difficulties  in  the  way  of  its  completion, 
as  well  as  in  its  entire  failure.  It  was  a  Government  work,  and 
the  object  was  to  construct  a  lighthouse  off  the  coast  of  North 
Carolina,  on  what  is  known  as  Diamond  Shoals.  The  Govern¬ 
ment  engineers  advertised  for  plans  and  estimates  of  cost, 
leaving  the  matter  of  design  and  methods  to  be  pursued  to 
those  desiring  to  bid  on  the  work.  The  result  was  that  three 
proposals  were  offered  by  American  builders,  differing  some¬ 
what  in  plans,  cost,  and  methods  of  procedure.  Owing  to  the 
exposed  location  of  the  structure  and  the  severe  and  sudden 
storms,  with  the  consequent  excessive  scouring  of  the  shifting 
sands,  the  greatest  difficulty  necessarily  arose  in  the  commence¬ 
ment  ;  and  the  success  of  the  enterprise  depended  mainly  on 
choosing  the  most  favorable  time  and  securing  a  good  hold  be¬ 
low  the  surface  between  the  periods  of  the  prevailing  stoims. 

One  of  the  plans  called  for  a  large  timber  caisson  sur¬ 
mounted  by  a  strong  double-walled  iron  crib  constructed  on 
top  of  it ;  the  spaces  between  the  walls  of  the  crib  to  be 
filled  with  concrete,  in  order  to  furnish  necessary  weight  to 
sink  the  caisson. 

Another  plan  in  which  both  caisson  and  crib  were  to  be 
constructed  principally  of  timber  and  concrete  used  in  the  cub 
to  sink  the  caisson.  In  both  cases  the  working  chambers  of 
the  caisson  were  to  be  filled  ultimately  with  conciete. 

The  third  plan,  which  was  accepted  by  the  engineers,  pro¬ 
vided  for  executing  the  work  by  the  open-crib  process,  with 
alternate  proposition  for  pneumatic  caisson,  if  found  necessaiy. 
This  can  better  be  described  in  the  words  of  the  contracting 
parties,  Messrs.  Anderson  &  Barr. 


CONSTRUCTION  OF  PNEUMATIC  CAISSONS.  289 


Diamond  Shoals  Lighthouse. 

Extracts  from  the  specification  of  Anderson  &  Barr,  con. 
tractors,  are  as  follows  : 

“  We  propose  to  sink  the  foundation  100  ft.  below  the  bed 
of  the  shoal,  if  the  material  in  the  way  of  sinking  the  founda¬ 
tion  is  such  that  we  can  remove  it  by  dredging.  If  the 
material  is  such  that  we  must  resort  to  the  use  of  compressed 
air  in  order  to  remove  it,  we  will  sink  the  caisson  80  ft.  below 
the  low-water  line,  unless  rock  is  encountered  before  that 
depth  is  reached.  The  foundation  to  consist  of  an  iron  caisson 
filled  solidly  with  cement  concrete.  Concrete  is  to  be  made  of 

1  part  of  Portland  cement,  2  parts  of  sand,  and  4  parts  of 
stone  broken  so  as  to  pass  through  a  2-in.  ring.  The  founda¬ 
tion  caisson  is  to  be  built  of  cast-iron  plates,  with  a  bottom 
section  of  wrought  steel.  The  total  height  is  155.5  ft.  The 
wrought-steel  bottom  section  is  of  cylindrical  shape,  54  ft. 
diameter  and  30  ft.  high.  On  top  of  this  is  bolted  a  cast-iron 
conical  section  20  ft.  high,  of  54  ft.  lower  and  45  ft.  upper  di¬ 
ameter.  On  top  of  this  is  placed  the  main  body,  which  is  of 
cylindrical  form  and  of  45  ft.  diameter,  and  which  continues  of 
even  shape  to  the  level  of  the  base  of  the  lighthouse  tower. 
Through  the  whole  body  of  the  caisson  parallel  with  its  axis 
pass  four  water-tight  steel  cylinders  of  9  ft.  diameter,  through 
which  the  ground  is  excavated  from  under  the  caisson. 

“At  19  ft.  high  from  the  bottom  edge  of  the  caisson  these 
cylinders  widen  out  into  irregular  conical  shapes,  which  end  at 

2  ft.  9  ins.  above  the  bottom  of  the  caisson  in  the  circumferen¬ 
tial  cylinder  shape  and  in  a  cross-bulkhead  of  3  ft.  height, 
which  divides  the  area  on  the  bottom  of  the  caisson  into  four 
equal  sectors  of  the  circle. 

Thus  for  2  ft.  9  ins.  height  the  bottom  of  the  caisson  con¬ 
sists  of  a  single  thickness  of  cylindrical  outside  plates,  and 
bulkheads  consisting  of  a  single  thickness  of  plate  which  divide 
the  area  of  the  circle  into  four  equal  parts,  each  of  which  is 
provided  centrally  over  it  with  an  open  vertical  tube  9  ft. 


29O  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


diameter,  for  the  purpose  of  dredging  its  quarter  compartment. 
The  bottom  circle  of  cylinder  plates  of  6  ft.  height  is  of  f-in. 
thickness.  The  cross-bulkhead  plates  are  of  f-in.  thickness. 
These  central  straight  plates,  as  well  as  the  outside  circle 
plates,  are  provided  with  stiffening  brackets  of  f-in.  thickness 
and  4-in.  angle-irons.  All  the  other  plates  of  the  wrought 
bottom  section  is  of  |--in.  thickness,  both  the  outside  cylinder 
and  the  interior  cones  and  tube  ends.  Four-inch  angle-irons, 
running  vertically  the  whole  length  of  the  section  and  4  ft. 
apart,  are  riveted  to  the  interior  of  the  cylinder,  and  corre¬ 
sponding  ones  to  the  conical  bottoms,  and  the  dredging  tubes 
are  braced  together  by  4-in.  angle-iron  bracing  in  such  form  as 
to  generally  stiffen  the  structure  against  both  outside  and  in¬ 
side  strains.  Similar  angle-irons  brace  those  portions  of  cones 
and  dredging  tubes  together  which  face  one  another.  The 
top  of  the  wrought  section  is  provided  with  a  6-in.  angle-iron 
on  the  inside  of  the  cylinder,  to  which  the  cast-iron  cone  is 
bolted.  This  cone,  as  well  as  the  cylindrical  portion  of  45  ft. 
diameter  above  the  cone,  consists  of  cast-iron  plates  of  i^-in. 
thickness,  and  of  such  horizontal  length  as  to  make  the  cir¬ 
cumference  of  20  plates  and  of  5  ft.  height.  The  plates  are 
provided  with  planed  flanges  forming  6-in.  depth  of  joint  all 
around  them,  strongly  bolted  together  with  i-in.  bolts  and 
nuts,  and  laid  so  that  the  vertical  seams  of  different  layers 
break  joints.  Lugs  are  cast  on  the  plates,  from  which  4-in. 
angle-iron  braces  run  to  the  nearest  of  the  four  central  tubes. 
These  latter  are  of  a  uniform  diameter  of  9  ft.,  made  of  -J-in. 
plate  in  sections  5  ft.  high,  like  the  outside  plates,  and  provided 
on  top  and  bottom  with  4-in.  angle-iron  outside  rims  for  join¬ 
ing  them  by  means  of  i-in.  bolts  and  nuts.  The  braces  from 
the  cast-iron  circumference  plates  are  attached  to  these  rims; 
also  the  braces  by  which  the  tubes  are  braced  to  one  another. 
For  convenience  of  transportation  and  erection  the  cylinders 
are  in  halves,  joined  by  4-in.  angle-irons  and  i-in.  bolts  on  the 
vertical  seams.  These  tubes  extend  up  to  2  ft.  above  the 
high-water  line,  and  above  that  point  the  circumference  plates 
are  braced  by  turnbuckle  bolts  of  ij-in.  diameter. 


CONSTRUCTION  OF  PNEUMATIC  CAISSONS. 


29I 


The  whole  interior  of  the  caisson,  including  the  tubes,  will 
be  filled  with  cement  concrete,  except  that  seven  cylindrical 
vaults  will  be  built  in  the  floor  of  the  towers.  On  the  top  sur¬ 
face  of  the  concrete  a  cast-iron  base  of  42^  ft.  outside  diameter 
and  2  ft.  width,  if-in.  thickness  will  be  placed,  on  which  the 
tower  will  be  erected. 

Total  weight  of  the  structure,  3,832,400  lbs. 

Concrete  about  10,000  cubic  yards,  and  contract  prices  for 
the  structure  completed  in  place,  $485,000. 

The  above  is  copied  from  the  columns  of  the  Engineering 
News. 

For  elevation,  vertical  section,  and  plan,  see  Figs.  I  and  2, 
Plate  X. 

29.  The  Cairo  bridge  across  the  Ohio  River,  near  its 
mouth,  was  constructed  in  1887-88  by  the  Union  Bridge  Co. 

The  superstructure  of  the  bridge  proper  consists  of  12 
single-track  steel  spans,  varying  in  length  from  249  ft.  to  518^ 
ft.  The  piers  supporting  the  longer  spans  rested  on  pneu¬ 
matic  caissons  sunk  75  ft.  below  low  water.  The  masonry 
started  25  ft.  below  low-water  and  10  ft.  below  the  bed  of  the 
river.  Length  between  end  piers,  4644^  ft.  The  approach  on 
the  Kentucky  side  consisted  of  21  spans  of  1 50  ft.  =  3 1 50  ft. 
and  4594  ft.  of  timber  trestle.  On  the  Illinois  side  the  ap¬ 
proach  consisted  of  17  spans  150  ft.  and  1  span  106J  ft.  =  2550 
ft.  and  5327  ft.  of  timber  trestle,  All  approach  spans  rested 
on  piers  composed  of  two  steel  cylinders  filled  with  concrete 
and  resting  on  piles. 

The  river  bed  is  alluvial  soil ;  some  loose  rock  was  found  at  a 
depth  of  175  ft.  It  was  determined  to  use  the  caissons  for  the 
foundations,  as  the  apprehension  of  encountering  logs,  wrecks, 
etc.,  rendered  the  use  of  the  open  crib  sunk  by  dredging  risky 
and  uncertain,  and  the  loose  nature  of  the  material  together 
with  the  rapid  currents  in  floods  precluded  the  use  of  piles. 
The  dimensions  of  the  caissons  were  30  X  70  ft.  and  26  X  60  ft. 
The  height  of  the  caissons  and  cribs  were  about  50  ft.  The 
caisson  proper  was  16  ft.,  the  pitch  of  the  working  chamber  8 
ft.,  with  two  courses  of  solid  timber  forming  the  deck  proper; 


292  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


and  on  top  of  this  six  courses  of  timber  of  open-work  crossing 
each  other,  and  on  this  34  ft.  of  open-work  crib,  similar  to 
the  upper  6  courses  of  what  is  called  the  caisson,  leaving  there¬ 
fore  12-inch  spaces  between  all  of  the  timbers  in  every 
direction,  except  that  the  cross-braces  divided  the  entire  crib- 
work  into  a  series  of  hollow  prisms  7  ft.  sq.,  extending  from 
the  top  to  the  solid  courses  of  the  deck.  The  outside  walls 
were  covered  with  two  courses  of  3-in.  oak  plank,  the  inner 
layer  placed  diagonally  and  the  outer  layer  vertically.  The 
walls  of  the  working  chamber  were  V-shaped  and  built  hollow. 
The  whole  was  tied  together  by  screw  and  drift  bolts  and 
spikes.  The  working  chamber  was  lined  with  3-in.  plank, 
caulked  and  painted  with  two  coats  of  white  lead  to  prevent 
air  leakage  and  aid  in  lighting  the  interior.  The  shoe  of  the 
caisson  was  made  of  iron  plates  f-in.  thick  and  36  ins.  deep. 
The  main  shaft  was  only  3  ft.  in  diameter;  supply  shaft  2  ft. 
The  air-lock  was  made  of  ^-in.  iron  plates  9  ft.  long,  6  ft.  wide, 
and  7  ft.  high,  with  circular  ends  3  ft.  radius,  and  was  divided 
into  compartments  forming  independent  locks,  and  was  placed 
8  ft.  above  the  deck  of  the  caisson.  The  usual  air,  water  and 
discharge  pipes  were  used.  The  sand  in  the  working  chamber 
was  removed  by  the  Monson  sand  pump,  somewhat  different 
in  design  from  the  mud  pump  to  be  described  presently,  but  sim¬ 
ilar  in  principle.  The  blowing-out  process  was  also  used,  and  to 
avoid  a  too  great  waste  of  air  when  the  material  was  blown  out 
dry,  the  pipe  rvas  extended  below  the  cutting-edge  so  as  to  be 
underwater.  The  maximum  sinking  in  24  hours  was  10.63  ft., 
but  the  usual  progress  in  clean  sand  was  from  2  to  4  ft.  daily  ; 
in  some  caissons  it  was  only  1.1  to  2  ft.  The  greatest  immer¬ 
sion  was  94.2  ft.  The  calculated  frictional  resistance  was  from 
597  to  715  lbs.  per  square  foot  of  surface  (the  estimated 
resistance  before  sinking  was  400  lbs.  per  square  foot)  at  a- 
depth  in  the  sand  of  86.42  ft.  After  several  cases  of  paralysis 
and  two  deaths,  a  warm,  comfortable  room  was  fitted  up, 
and  also  hot  baths  and  coffee  were  provided,  after  which 
no  further  serious  illness  occurred.  The  temperature  of  the  air 
in  caisson  was  also  cooled  by  passing  through  coils  of  pipe  kept 


CONSTRUCTION  OF  PNEUMATIC  CAISSONS. 


293 


surrounded  with  cool  water,  lowering  the  temperature  from 
1250  to  90°.  The  time  of  working  the  men  varied  from  8 
hours  to  1^  hours  per  shift,  allowing  from  16  to  21  hours  of 
rest ’during  the  24  hours.  Portland  cement  concrete  was  made, 
I  cement,  2  sand,  3  broken  stone.  Louisville  cement  concrete, 
1  cement,  2  sand,  3§  broken  stone.  The  piers  for  the  ap¬ 
proach  spans  consisted  of  two  cylinders  8  ft.  diameter,  placed 
18  ft.  centres,  braced  together.  Metal  thickness  £-in.  plates, 
spliced  on  the  inside.  A  pit  was  excavated  8  ft.  deep,  in 
the  bottom  of  which  twelve  oak  piles  were  driven  ;  the  pits 
were  then  partly  filled  with  concrete,  and  the  cylinders  placed 
on  the  concrete.  Concrete  was  then  packed  around  the  cylinders 
below  the  surface  and  also  in  the  cylinders  to  the  top,  and  was 
left  about  in.  above.  Over  the  top  a  steel  plate  \  in.  thick  was 
placed.  After  allowing  400  lbs.  per  square  foot  of  surface  on 
the  caissons  for  frictional  resistance,  and  after  deducting  the 
buoyant  effect  of  the  water  and  sand  (respectively  22,756  cu. 
ft.  and  78,000  cu.  ft.)  amounting  to  9544!  tons,  from  the  total 
weight  of  15,865.9  tons,  the  estimated  weight  on  the  founda¬ 
tion-bed  was  6291.4  tons,  or  3  tons  =  6000  lbs.  per  square  foot. 

The  precautions  taken  for  the  safety  and  comfort  of  the  men 
certainly  are  to  be  highly  commended.  The  writer  hesitates  to 
criticise  the  constructions  of  men  of  so  much  experience,  knowl¬ 
edge,  and  skill;  but  he  thinks  that  cutting  up  the  space  in  the 
cribs  with  timbers  in  such  numbers  separated  by  only  12  ins.  of 
space  is  a  faulty  construction,  and  must  necessarily  require  either 
a  great  deal  of  labor  and  care  to  fill  around  and  under  so  many 
square  timbers, — round  logs  for  cross-braces  would  to  some  ex¬ 
tent  remedy  the  objection, — or  if  the  work  is  carelessly  done 
there  must  exist  many  hollows  and  open  spaces.  The  position 
of  the  air-lock  near  the  bottom  can  hardly  be  recommended.* 


*  Only  the  caisson  proper  is  shown  in  Plate  XVIII.  The  roof  consists  of  two 
solid  courses  of  timbers,  and  six  courses  of  timbers  built  open,  as  shown  in  Fig.  3. 
The  crib  can  be  built  on  top  of  the  caisson  to  any  desired  height,  34  ft.  in  this 
case,  and  is  built  open,  as  shown  in  Fig.  3.  The  V-shaped  walls  can  be  built 
hollow  and  filled  with  concrete,  or  they  can  be  solid  built  with  timber,  as  shown 
on  the  right  in  Fig.  3. 


294  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


These  parties,  however,  have  put  in  more  caissons  than  almost 
any  others.  The  success  which  has  attended  their  works  cer¬ 
tainly  cannot  be  criticised,  and  it  must  be  presumed  that  they 
consider  it  economical  and  satisfactory  in  every  respect.  See 
Figs,  i,  2,  3,  4  and  5,  Plate  XVIII. 

29^.  Having  described  briefly  the  caissons  of  the  above 
large  bridges,  we  will  now  consider  in  somewhat  greater  detail 
the  design  and  construction  of  the  caissons  used  by  the  writer 
on  several  large  bridges,  as  the  Susquehanna  Bridge,  Havre 
de  Grace,  Md.,  the  Schuylkill  Bridge,  Philadelphia,  Pa.,  and 
the  Tombigbee  River  Bridge,  Ala.  The  caissons  were  nearly 
as  large,  and  the  depths  sunk  were  about  as  great ;  hence  the 
details  of  construction,  methods  of  sinking,  etc.,  would  be 
equally  applicable  to  any  of  those  already  described  with  few 
modifications  of  minor  importance,  while  in  connection  with 
these  the  descriptions  will  be  based  upon  actual  experience,  as 
the  caissons  were  designed  by,  and  the  work  executed  under  the 
direct  supervision,  of  the  writer;  but  as  applied  to  the  others  they 
would  be  purely  a  compilation  from  the  descriptions  of  others. 

The  design  and  construction  of  the  caissons  were  the  same 
for  the  three  bridges.  There  were  5  caissons  in  the  Susquehanna 
River  Bridge,  varying  in  dimensions  from  63.27  X  25.93  ft.  to 
78.19  X  42.27  ft.  and  a  general  thickness  of  roof  of  8  ft.  The 
widest  caisson  had  a  roof  10  ft.  thick  ;  these  were  built  solid,  of 
courses  of  12  x  12  in.  pine  timber.  At  the  Schuylkill  there 
were  two  rectangular  caissons  65.5  X  23.5  ft.,  one  octagonal 
caisson  50  ft.  in  diameter  of  circumscribing  circle  for  pivot 
pier,  and  one  nearly  square  caisson  44  X  45  ft.  for  a  U-abut- 
ment ;  the  roof  of  this  latter  was  10  ft.  thick,  of  the  others 
8  ft.  thick — the  depths  sunk  varying  from  40  to  90  ft.  below 
low-water. 

At  the  Tombigbee  Bridge  there  were  two  rectangular 
caissons  45  X  23  ft.  and  one  octagonal  caisson  24  ft.  diameter 
for  the  draw  pier.  These  caissons  were  sunk  only  about  33  ft. 
below  low-water,  but  the  excavation  was  continued  about  9  ft. 
below  the  cutting-edge  of  the  caisson  to  a  point  about  42  ft. 
below  low-water  and  82  below  high-water. 


CONSTRUCTION  OF  PNEUMATIC  CAISSONS.  295 


30.  On  all  of  these  caissons  cribs  were  constructed  and  filled 
with  concrete,  varying  in  height  from  20  to  40  ft.  of  the  same 
horizontal  sections  as  the  caissons  at  the  top,  which  was  about 
20  ins.  less  in  each  dimension  than  that  given  above,  as  the 
caissons  were  15  ft.  high,  and  had  a  batter  all  around  of  in. 
to  each  vertical  foot. 

31.  Coffer-dams  were  constructed  on  top  of  the  cribs  from 
20  to  40  ft.  high,  according  to  the  depth  of  the  water  in  which 
the  masonry  of  the  piers  was  constructed. 

32.  A  description  of  one  caisson,  crib,  and  coffer-dam  will 
answer  for  all,  with  a  few  modifications  for  the  octagonal  forms 
required  by  its  shape.  The  descriptions  will  be  better  and 
more  easily  understood  by  reference  to  Plates  XIII,  XIV,  XV, 
XVI,  and  XVII.  The  plates  show  horizontal  and  vertical 
sections,  plans,  details,  etc.,  of  caissons,  cribs,  and  coffer-dams. 
The  caisson  was  constructed  by  first  building  a  solid  wall 
of  five  or  six  courses  of  timber,  12^  X  12  ins.  cross-section, 
surrounding  the  required  space,  and  built  with  the  proper 
batter.  On  the  outside  of  this,  timbers,  12  X  14  ins.  X  14  ft., 
were  placed  all  around,  extending  2  ft.  below  the  timber- 
wall  and  6  ft.  above  ;  the  lower  edges  of  these  pieces  were  cut 
to  a  bevel,  the  lower  cutting-edge  being  3  inches  thick.  On 
the  inside  of  the  wall  three  courses  of  3-in.  plank  were  placed, 
crossing  each  other  diagonally,  and  on  the  inside  of  this  a 
single  course  placed  vertically — for  convenience  of  calking. 
The  courses  of  plank  were  cut  to  a  level  with  the  top  of  the 
wall  and  reached  to  within  one  foot  of  its  lower  edge.  The 
whole  was  then  bolted  together  by  both  screw  and  drift- 
bolts,  as  shown  in  the  drawings.  Each  layer  of  plank  was 
also  spiked  with  two  spikes,  5^  inches  long,  to  each  lineal 
foot  of  plank.  Then  plank  was  also  spiked  in  one  layer  on  all 
interior  surfaces  below  the  courses  of  plank  above  mentioned. 
This  completed  the  walls  of  the  working  chamber.  The  deck 
courses  were  then  placed  between  the  verticals  projecting  up¬ 
ward  and  resting  on  top  of  the  timber-wall  and  the  four 
courses  of  plank  on  the  inside,  which  gave  a  2-ft.  bearing  on 
all  sides.  The  arrangement  of  the  courses  was  as  follows: 


296  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


1st.  A  course  of  timbers  in  one  length,  laid  transversely;  2d,  a 
course  diagonally,  of  varying  length  ;  3d,  another  course  laid 
transversely,  of  single  length.  At  the  top  of  this  course  a  2-in. 
shoulder  was  formed  in  the  verticals  ;  4th,  a  course  laid  longi¬ 
tudinally,  resting  on  the  shoulder  ;  5th,  another  transverse 
course,  in  single  lengths  ;  6th,  a  diagonal  course  in  varying 
lengths ;  the  verticals  were  cut  off  on  a  level  with  the  top  of 
this  course.  The  7th  and  8th  courses  were  transverse  and  of 
single  length,  reaching  from  out  to  out  over  the  tops  of  the 
verticals.  These  latter  were  bolted  by  screw  and  drift-bolts 
to  the  deck-courses,  as  shown  on  the  drawings.  The  deck- 
courses  were  all  bedded  in  a  good  bed  of  cement-mortar  and  a 
thin  grout  poured  into  the  intervals  between  the  timbers  of 
the  same  course  ;  this  interval  being  about  f  inch.  Each  stick 
was  bolted  to  the  course  below  by  drift-bolts,  1  in.  square  or 
round,  at  intervals  of  about  5  feet.  The  underside  of  the  roof 
was  lined  with  3-in.  plank.  The  whole  interior  was  then  thor¬ 
oughly  calked  with  oakum.  This  extended  the  full  thickness 
of  the  plank,  and  when  properly  done  the  oakum  compressed 
would  be  harder  than  the  timber  itself.  The  ends  of  all  bolts 
and  spikes  were  also  covered  or  wrapped  with  oakum,  the 
heads  and  nuts  bearing  hard  against  the  oakum.  The  shafts, 
pipes,  etc.,  were  built  into  the  roof,  all  spaces  around  them 
filled  with  mortar.  In  all  caissons  a  longitudinal  truss  was 
constructed,  resting  on  and  fastened  by  iron  straps  and  bolts 
to  the  end  walls.  This  truss  was  about  6  feet  deep,  the  upper 
and  lower  chords  composed  of  two  pieces  12X12  in.  timbers. 
The  web-members,  both  vertical  and  diagonals,  were  com¬ 
posed  of  timber  struts,  and  diagonal  rods  if  in.  in  diameter, 
these  latter  extending  through  the  first  deck-course.  This 
truss  formed  a  strong  stiffening  rib  for  the  roof,  and  also 
braces  for  the  end-walls,  and  in  addition  affording  a  broad 
bearing  surface  for  blocking  or  for  the  earthy  material.  Cross¬ 
braces  were  placed  between  the  bottom  chord  of  the  truss  and 
the  side-walls.  These  were  of  timber,  either  12  X12  ins.  or 
12  X  16  ins.,  depending  upon  the  length  required.  In  addi¬ 
tion,  at  each  strut-brace,  iron  rods,  2  ins.  diameter,  with 


CONSTRUCTION  OF  PNEUMATIC  CAISSONS.  297 

swivels,  extended  across  the  caisson  and  through  the  side- 
walls.  Details  of  these  rods  are  given  in  Plate  XVII. 

33.  This  completed  the  caisson  proper.  The  caissons  were 
built  partly  on  shore,  supported  5  or  6  feet  above  the  ground 
on  blocks  of  timber.  Generally  only  one  or,  at  most,  two 
courses  of  deck-timbers  were  placed,  until  the  caisson  was 
launched.  After  the  interior  was  completed  and  calked, 
launching-ways  were  built  under  the  caisson,  and  the  caisson 
supported  on  a  number  of  screw-jacks  ;  the  cradles  or  sliding- 
ways  were  adjusted,  the  jacks  lowered,  so  as  to  let  the  caisson 
rest  on  the  cradles,  and  when  everything  was  ready  the  cais¬ 
son  was  launched,  then  floated  to  its  proper  position,  where  it 
was  completed.  It  is  not  necessary  to  put  a  bottom  to  the 
caisson  when  deep  water  is  accessible ;  it  causes  ultimate 
delay  and  trouble  to  remove  it.  The  caissons  finished  as 
above  described,  with  only  2  deck-courses,  would  be  im¬ 
mersed  only  about  8  or  9  feet.  With  a  good  bottom,  they 
would  float  in  about  3  or  4  feet  of  water.  Plates  XIV,  XVII, 
and  XV,  Figs.  1,  2,  and  3,  show  details,  section,  and  plan  of 
an  octagonal  caisson,  the  interior  struts  radiated  from  a  centre- 
post  and  rested  against  the  sides  ;  the  iron-rods  radiated  from  a 
centre-collar  of  iron,  and  passed  through  the  angles.  In  all 
the  caissons,  iron-bars,  2^  X  1  in.  X  8  or  10  feet,  were  bent 
around  the  angles  on  the  outside  and  bolted  to  the  timbers,  3 
or  4  straps  being  placed  at  each  corner. 

34.  The  advantages  of  these  forms  of  caissons  are  evident. 
The  walls  of  the  working  chamber  are  strongly  and  firmly  con¬ 
nected  with  the  roof  of  the  caisson,  forming  stout  cantilevers, 
thereby  relieving  the  pressure  on  the  braces,  as  was  evidenced 
by  the  fact  that  in  no  case  were  the  wedges  at  the  ends  of  the 
strut-braces  in  working  chambers  crushed,  even  under  very  try¬ 
ing  circumstances,  as  when  the  air  escaped  suddenly  from  the 
caisson  ;  and  all  parts  acting  together  local  and  excessive  strains 
never  caused  any  springing  or  leaks,  nor  was  there  any  creaking 
or  cracking  of  timbers  to  alarm  the  men.  The  walls  of  the 
working  chamber  are  so  constructed  that  the  men  had  easy 
access  to  the  cutting  edge,  and  at  the  same  time  broad  hori- 


298  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


zontal  surfaces  are  provided  so  as  to  obtain  many  square  feet 
of  bearing  surfaces  at  any  part  of  the  caisson  or  entirely  around, 
in  addition  to  the  bearing  surfaces  under  the  centre  truss. 
These  conditions  are  of  great  advantage  in  many  cases.  The 
caisson  can  be  better  kept  level,  or  in  case  of  careening  can  the 
’more  easily  be  brought  to  a  level,  and  there  is  less  danger  of 
settling  until  everything  is  ready,  as  the  material  can  be  left 
under  these  bearing  surfaces ;  and  in  short  the  caisson  can  be 
controlled  and  regulated  much  better  than  when  the  walls  of 
the  working  chamber  are  V-shaped.  The  latter  design  of 
caisson  is  shown  in  Plate  XVIII,  Figs.  1,  2,  3,  4,  and  5. 

35.  The  only  accident  that  happened  during  the  construc¬ 
tion  of  the  Susquehanna  bridge  was  caused  by  the  neglect  of 
one  of  the  foremen,  and  as  much  can  be  learned  from  accidents 
this  one  will  be  briefly  described.  The  largest  caisson  had 
reached  a  point  within  seven  feet  of  the  rock  at  its  highest 
point,  but  was  twenty-eight  feet  above  the  rock  at  its  lowest 
point.  Owing  to  the  softness  of  the  material  through  which  we 
were  sinking,  it  was  necessary  to  stop  concreting  in  the  crib  to 
avoid  too  much  weight ;  the  coffer-dam  had  been  constructed 
on  the  crib,  but  had  not  been  braced  on  the  interior.  At  this 
time  the  top  of  the  crib  was  only  a  few  inches  above  the  surface 
of  the  water  ;  the  pockets  of  the  crib  were  empty  for  a  consid¬ 
erable  depth.  Without  observing  these  conditions  the  foreman, 
being  ready  to  sink  the  caisson,  lowered  the  pressure  at  a  time 
when  the  tide  was  at  its  highest.  As  the  caisson  settled  the 
water  raised  a  few  feet  above  the  crib  ;  the  pressure  caused  one 
side  of  the  coffer  dam  to  be  forced  inwards,  the  water  rapidly 
filling  the  crib  and  adding  about  14,000,000  lbs.  of  weight;  the 
caisson  sank  suddenly  until  one  end  rested  on  the  rock,  then 
careening,  settled  at  the  other  end  until  a  sufficient  bearing  on 
the  roof  of  the  caisson  stopped  it.  Seven  men  were  in  the 
chamber  at  the  time;  they  were  fortunately  either  in  the  shaft 
or  near  to  it,  and  ascended  to  a  place  of  safety;  fortunately 
the  lower  door  was  closed  at  the  time,  and  they  could  not  enter 
the  lock.  The  upper  end  of  the  shaft  sank  under  the  water, 
allowing  the  air-lock  to  be  filled  with  water;  pipes  were  broken 


CONSTRUCTION  OF  PNEUMATIC  CAISSONS. 


299 


off,  and  leaks  were  caused  in  the  main  shafts.  This  air  following 
the  shaft  made  the  water  boil  up  furiously  around  the  top  of  the 
lock.  Four  large  air  compressors  were  started  at  once,  all  pipes 
were  plugged  up  above  the  surface ;  and  notwithstanding  the 
large  quantity  of  air  that  was  being  forced  into  the  caisson,  the 
leaks  in  the  shafts  were  so  great  that  the  water  was  gradually 
rising.  The  men  tore  their  clothes  and  stuffed  them  in  the 
openings.  Great  difficulty  was  encountered  in  getting  a  dam 
around  the  mouth  of  the  shaft ;  but  by  the  use  of  planks,  tar¬ 
paulins,  etc.,  the  bubbling  and  boiling  was  stopped,  and  a  dam 
of  cement  in  bags  was  made,  the  interstices  packed  with  oakum 
dipped  in  mortar.  We  at  last  were  able  to  bail  the  water  out  of 
the  lock,  and  the  men  were  released,  after  having  been  confined 
in  their  perilous  position  for  about  eight  hours.  When  the 
necessary  repairs  were  made  and  the  air  again  forced  into  the 
caisson,  it  was  found  that  no  leaks  existed  in  the  caisson  proper  ; 
that  the  only  damage,  outside  of  broken  valves,  pipes,  etc.,  had 
occurred  where  the  caisson  had  brought  up  hard  on  the  rock ; 
the  lower  ends  of  the  verticals  had  been  crushed  off  to  a  height 
of  about  two  feet  around  one  corner  of  the  caisson.  This  was  of 
no  moment,  and  the  work  proceeded  at  once  The  rock  was 
blasted  to  a  depth  sufficient  to  level  the  caisson,  which  was  ac¬ 
complished  without  any  special  trouble.  We  can  learn  from 
this,  1st.  That  the  caisson  should  not  be  sunk  until  a  careful 
examination  is  made  to  see  that  everything  is  ready;  2d.  Al¬ 
ways  keep  the  top  of  the  main  shaft  well  above  the  surface  of 
the  water ;  3d.  Always  bring  the  men  out  of  the  caisson  before 
sinking  the  caisson,  and  4th.  The  necessity  of  having  ample 
steam-producing  capacity  and  also  reserve  compressors,  to  sup¬ 
ply  large  quantities  of  air,  connected  up  and  ready  for  work  at 
a  moment’s  notice.  Anything  short  of  this  amounts  to  gross 
negligence  or  carelessness.  Air  compressors  are  often  stopped 
for  a  greater  or  less  time  (and  men  left  in  the  chamber)  for 
small  repairs,  want  of  steam,  or  other  causes,  or  in  case  of  any 
alterations  in  the  air  connections  of  pipes  or  shafts.  The  men 
should  invariably  be  taken  out  of  the  caisson ;  many  lives  have 
been  lost  in  recent  years  by  a  failure  to  do  so. 


300  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


36.  In  some  large  bridges  the  masonry  is  commenced  on  top 
of  the  caisson,  without  using  cribs  or  even  coffer-dams  ;  there 
is  always  danger  of  delay  and  extra  cost.  The  crib  can  be  dis¬ 
pensed  with,  but  a  coffer-dam  should  always  be  constructed,  as 
it  is  vastly  cheaper  to  construct  a  dam  while  the  work  pro¬ 
gresses  than  to  build  one  after  the  deck  of  the  caisson  has  dis¬ 
appeared  under  water. 

37-  A  crib  is  only  necessary  for  rapidity  of  sinking,  and  is  a 
matter  of  economy ;  a  timber  or  iron-cased  crib  will  not  cost 
more  than  one  half  to  one  third  that  of  masonry  per  cubic 
yard  ;  the  crib  can  be  built  up  rapidly  and  with  relatively  small 
expense  in  calking  will  be  sufficently  water-tight ;  occasional 
bailing  or  pumping  will  keep  the  leaks  down.  A  crib  of  this 
kind  is  really  a  solid  single-wall  coffer-dam,  well  braced  on  the 
interior. 

Sometimes  cribs  are  built  open,  leaving  12- in.  spaces 
between  the  courses  of  timbers;  the  transverse  braces  passing 
between  the  courses  having  likewise  12-in.  spaces  between  them. 
There  is  no  economy  in  this,  as  the  spaces  are  to  be  filled 
with  concrete  (see  Plate  XVIII).  As  water  circulates  freely  in 
the  crib,  much  of  the  concrete  will  either  have  to  be  placed 
under  water  or  subjected  to  the  action  of  the  water  before  it 
has  had  time  to  set,  preventing  sound,  solid  work  and  causing 
waste  of  good  material.  The  work  cannot  be  altogether  sat¬ 
isfactory,  and  as  the  concrete  should  be  packed  under  and 
between  the  cross-timbers,  hollow  spaces  will  necessarily  exist. 
This  can  be  avoided  largely  by  using  round  logs,  stripped  of 
bark,  for  the  cross-braces  ;  they  are  equally  as  good,  and  would 
cost  somewhat  less  than  braces  sawed  square.  A  crib  thus 
constructed  practically  divides  the  mass  of  concrete  into  iso¬ 
lated  prisms  or  columns  of  concrete. 

Solid  walls,  either  calked  or  not,  are  used  with  solid  or  open¬ 
work  cross-walls,  or  braces ;  the  same  objections  occur  in  this 
case,  so  far  as  isolating  the  columns  of  concrete. 

In  the  cribs  used  in  the  structures  now  being  described, 
both  of  these  objections  were  to  a  great  extent  removed.  The 
outside  walls  were  built  solid  and  calked  ;  the  cross-walls  were 


CONSTRUCTION  OF  PNEUMATIC  CAISSONS. 


301 


built  solid  for  a  few  courses  of  a  height  one  third  or  one 
fourth  that  of  the  crib.  The  positions  of  the  cross-walls  were 
then  shifted  to  the  middle  of  the  pockets  below,  and  built  up 
solid  for  a  like  height,  then  shifted  vertically  over  the  lower 
walls,  and  so  on  alternating  2  and  3,  and  3  and  4  in  number. 
In  this  case  it  was  only  necessary  to  pack  the  concrete  under  a 
few  timbers,  which  could  be  cut  on  a  bevel  or  be  round ;  the 
various  columns  of  concrete  were  consolidated  into  practically 
a  homogeneous  and  united  mass.  The  water  could  be  kept 
from  any  layer  as  long  as  desired.  The  side-walls  were  dove¬ 
tailed  at  the  corners,  and  the  cross-walls  were  dovetailed  into 
the  timbers  of  the  outside  walls.  All  timbers  were  drift-bolted 
together  with  i-in.  round  or  square  bolts  22  ins.  long,  spaced 
about  5  ft.  intervals — the  courses  of  timber  breaking  joints. 
These  cribs  were  planked  on  the  outside,  the  plank  placed  ver¬ 
tically  in  lengths  of  5  to  7  ft.  and  spiked ;  this  kept  the  calk¬ 
ing  in  place — otherwise  of  no  special  advantage.  At  the  5th  or 
6th  course  of  timber  from  the  top,  iron  bolts  2-ins.  diameter, 
with  a  large  eye  on  one  end,  were  placed  through  the  outer 
walls  of  the  crib  ;  these  were  for  connecting  the  vertical  rods  of 
the  coffer-dam.  Drawings,  Plate  XIII,  fully  illustrate  the  con¬ 
struction  of  the  cribs ;  these  were  square-ended.  Pointed-end 
cribs  can  easily  be  constructed  when  desired,  and  should  be 
pointed,  if  they  extend  near  to  the  surface  of  the  water,  so  as 
not  to  obstruct  the  current  too  much  (see  Plate  XIX).  The 
tops  of  these  cribs  were  from  15  to  30  or  more  feet  below  low- 
water,  and  did  not  justify  the  additional  amount  of  material  and 
costs  as  it  would  have  also  necessitated  longer  caissons  and  con¬ 
siderable  extra  expense  in  the  sinking. 

38.  The  design  of  coffer-dams  used  was  simple,  strong,  and 
efficient.  When  the  height  required  was  over  20  ft.,  they  were 
built  in  two  sections  similar  in  every  respect  to  each  other.  A 
12  X  12  in.  sill  was  placed  on  the  walls  of  the  crib,  overlapping  it 
by  3  ins.  on  the  inside.  Vertical  pieces  12X12  ins.  were  erected 
on  these  at  intervals  of  4  or  5  ft.,  connected  with  the  sill  by 
mortise  and  tenon,  and  then  caps  mortised  and  tenoned  to 
them ;  cross-pieces  placed  across  the  top  projected  outward 


302  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 

over  the  iron  eye-bolts  in  the  crib  and  long  iron  rods  2  ins. 
diameter  with  hooks  at  one  end  and  threads  at  the  oth&r  were 
hooked  to  the  eye-bolts  and  passed  through  holes  in  the  cross¬ 
pieces,  on  the  upper  ends  thimbles  or  sleeves  with  right  and 
left  hand  threads  were  screwed,  pressing  the  coffer-dam  hard 
to  the  crib.  The  sleeves  were  used  instead  of  nuts,  so  as  to 
connect  other  rods  for  the  upper  sections  of  the  dams  ;  the 
usual  nuts  were  used  on  the  top  of  these  bolts.  A  double 
course  of  3-in.  plank  was  then  spiked  to  the  uprights — the  first 
or  inner  course  laid  diagonally,  and  the  outer  horizontal  for 
convenience  of  calking  ;  the  entire  outside  was  well  calked. 
Two  tiers  of  braces  on  the  inside  were  sufficient  for  a  section  20 
ft.  high  ;  strong  cleats  were  spiked  to  the  cross-walls  of  the  crib 
to  brace  the  bottom  sills.  Plans,  sections,  and  details  are  shown 
in  Plates  IV,  Figs.  1  and  2,  and  XIII,  Figs.  1,  2,  and  4.  The 
upper  section  was  constructed  in  the  same  manner.  This  con¬ 
struction  answers  well  for  heights  from  40  to  45  ft.  The  octag¬ 
onal  cribs  and  coffer-dams  are  different  only  in  the  struts  and 
rods  for  bracing  which  radiate  from  centre-posts.  The  corner- 
posts  of  all  dams  were  made  in  two  pieces,  and  bolted  together  ; 
when  these  bolts  were  removed  and  the  iron  rods  tinhooked, 
the  sides  and  ends  were  free  to  separate.  It  was  intended  to 
use  these  on  other  cribs,  but  it  did  not  prove  either  economi¬ 
cal  or  practicable.  The  only  coffer-dam  that  gave  away  was 
caused  by  the  accident  already  explained. 

Plate  XVIII,  Figs.  1,  2,  and  3,  shows  another  form  of 
caisson,  crib,  etc.,  often  used,  open-wall  cribs  being  employed. 

Article  L. 

CAISSON  SINKING. 

39.  The  construction  of  the  caissons,  air-locks,  size,  and 
kind  of  pipes,  machinery,  connections,  etc.,  having  been 
described,  it  only  remains  to  explain  briefly  the  methods  used 
in  excavation,  sinking  caissons,  and  filling  the  working  chamber 
with  concrete.  It  may  be  stated  that  in  general  all  materials 
that  are  too  large  to  pass  out  through  the  pipes  have  to  be 


CAISSON  SINKING. 


303 


carried  out  through  the  main  shaft  in  buckets  or  bags.  There 
are  patent  buckets,  which  slide  through  a  shaft  left  in  the 
caisson,  being  raised  or  lowered  by  machinery.  When  the 
bucket  is  lowered  into  the  working  chambers  by  the  proper 
adjustment  of  valves  and  pipes,  doors  can  be  opened  into  the 
working  chamber,  large  bowlders,  sticks  of  wood,  and  other 
debris  can  be  thrown  into  the  bucket,  the  doors  closed,  air 
pressures  equalized,  and  the  bucket  with  its  load  lifted-  out. 
The  arrangement  is  simple  and  efficient,  but  has  never  been 
generally  adopted.  In  the  writer’s  experience  the  larger 
bowlders  and  pieces  of  crushed  rock  were  generally  piled  on  plat¬ 
forms  resting  on  the  truss  and  braces  and  carried  down  with 
the  caisson,  mainly  removing  from  the  interior  the  sand, 
gravel,  etc.  The  bowlders  were  ultimately  used  in  the  concrete 
or  rubble-work  in  the  chamber.  There  are  some  objections  to 
this,  as  the  men  are  inconvenienced  in  moving  about,  and  have 
to  work  under  heavily  loaded  platforms,  which  involve-s 
some  danger.  It  causes  some  delay  and  expense,  but  on  the 
whole  is  probably  more  economical  than  breaking  up  the 
bowlders,  removing  them  from  the  caissons,  and  again  putting 
them  back  in  the  form  of  concrete. 

40.  The  removal  of  the  sand,  gravel,  and  mud  can  be  ef¬ 
fected  by  the  sand  pump,  mud  pump,  or  by  the  blowing-out 
process,  each  of  which  will  be  briefly  described.  As  has  been 
mentioned,  a  number  of  discharge  pipes  were  built  into  the 
caisson,  extending  through  the  deck.  Sections  of  pipe  8  or  9 
ft.  long  are  screwed  on  to  these  at  the  bottom,  reaching  down 
into  the  material,  the  lower  end  bent  at  right  angles.  A  small 
wooden  paddle  is  pressed  against  the  end  by  the  air  when  the 
valve  is  open  ;  the  material  excavated  is  shovelled  in  a  pile 
around  the  lower  end  of  the  pipe  ;  when  the  paddle  is  removed 
the  air  forces  the  material  up  and  through  the  pipe  with  great 
force.  At  the  top  an  elbow  or  goose-neck  of  chilled  iron  two 
or  three  inches  thick  is  fastened.  This  discharges  the  ma¬ 
terial  outward  and  downward.  These  elbows  are  rapidly  cut 
through  by  the  sand  and  gravel,  requiring  frequent  renewals. 

For  details  of  shafts,  air-locks,  pipes,  etc.,  see  Plate  XX. 


304  A  practical  treatise  on  foundations. 

The  process  is  simple,  but  requires  great  care  in  feeding  the 
material  to  the  pipe  to  prevent  its  choking  up.  A  dense  fog  al¬ 
ways  sets  up  when  the  pressure  is  lowered,  and  often  water  rises 
in  the  caisson,  and  much  air  is  used,  taxing  the  air  compressors 
greatly.  Notwithstanding  these  objections  it  is  largely  used. 

41.  The  mud  pump  and  sand  pump  do  not  differ  materially 
in  design,  nor  at  all  in  principle.  The  mud  pump  will  be 
alone  described.  It  consists  of  a  pear-shaped  cast-iron  vessel 
about  15  ins.  in  diameter  and  length,  which  has  a  hemispherical 
lining,  a  a,  connected  with  the  top;  also 
three  openings  into  it ,b,  c,  and  d,  to  which 
hose  or  pipes  can  be  connected,  bg  is 
called  the  suction  pipe,  ch  the  supply 
pipe,  and  dk  the  discharge  pipe,  bg  is  a 
long  hose,  so  as  to  be  moved  freely  about ; 
its  lower  end  has  an  iron  strainer  to  pre¬ 
vent  any  large  material  or  sticks,  etc.,  en¬ 
tering.  Its  upper  end  is  screwed  into  the 
bell  and  has  a  hollow,  conical-shaped  point, 
which  reaches  into  the  neck  of  the  dis¬ 
charge  pipe,  which  also  tapers  slightly,  so 
that  the  annular  space  between  the  two 
can  be  either  widened  or  narrowed.  Water 
is  forced  by  a  large  pump  down  through 
the  supply  pipe,  and  impinging  on  the  iron 
lining  is  scattered  around  it,  and  then 
passes  upward  through  the  annular  space, 
and  upward  in  the  discharge  pipe,  dk. 

This  creates  a  partial  vacuum  at  the  end  of 
the  supply  pipe.  The  sand,  mud,  and  water  are  thus  drawn  up 
into  the  discharge  pipe,  and  are  discharged  at  the  top.  A  large 
quantity  of  material  can  thus  be  removed  without  decreasing 
the  air-pressure,  but  the  material  is  required  to  be  cut  up 
fine  and  mixed  with  water.* 

*  The  Monson  sand-pump  has  a  vertical  section  almost  oval  is  made  of 
cast  iron  with  wrought-iron  bands,  horizontal  section  is  nearly  circular.  The 
supply-pipe  enters  the  pump  near  the  bottom;  there  is  no  inner  lining,  other¬ 
wise  the  design  and  construction  is  similar  to  the  above-described  mud-pump. 


for  Removing  Material  from 
Working  Chambers  of  Pneu¬ 
matic  Caissons. 


CAISSON  SINKING. 


305 


42.  In  making  the  excavation,  the  material  should  not  be 
removed  from  under  the  shoulders  until  the  middle  space  has 
been  excavated  to  the  depth  of  2  or  more  feet  below  the  cut¬ 
ting-edge,  so  as  not  to  leave  the  caisson  unsupported  for  any 
great  length  of  time,  and  not  at  all  under  the  lower  side,  if 
the  caisson  is  out  of  level.  When  everything  is  ready,  the  men 
should  be  brought  out  and  the  caisson  lowered  by  gradually 
reducing  the  pressure.  When  the  resistance  to  lowering  is  very 
great,  requiring  a  great  reduction  of  pressure,  one  man  gener¬ 
ally  remains  in  to  see  if  any  serious  leaks  occur  or  any  great 
inflow  of  material  takes  place,  so  as  to  signal  for  the  pressure 
to  be  put  on.  He  could  readily  ascend  the  shaft  if  necessary. 

43.  As  has  been  stated,  the  borings  indicated  a  very  great 
difference  of  level  in  the  rock-bed,  being  from  15  to  20  ft.  in 
the  length  of  the  caisson  in  some  instances,  and  that  blasting 
from  the  surface  had  proved  impracticable  at  any  reasonable 
cost.  During  the  sinking,  as  the  caisson  approached  the  high¬ 
est  part  of  the  rock,  constant  soundings  were  made  with  an 
iron  rod,  to  avoid  the  danger  of  coming  suddenly  on  the  rock 
at  any  point,  and  when  the  highest  point  of  rock  was  reached 
our  principal  difficulties  commenced.  There  was  but  two 
courses  open  :  either  to  blast  the  rock  and  sink  the  caisson  to 
the  level  of  the  lowest  point  of  the  rock,  or  to  hold  the  caisson 
where  it  was  and  carry  on  the  excavation  below  the  cutting- 
edge,  then  build  up  with  concrete  under  the  caisson,  and 
then  fill  the  working  chamber.  The  latter  plan  was  adopted, 
as  the  rock  was  very  hard  and  only  small  charges  could  be 
used,  which  would  have  required  a  long  time  and  added  enor¬ 
mously  to  the  cost  of  the  work.  The  great  danger  in  excavat¬ 
ing  below  the  cutting-edge  arose  from  the  fear  of  the  caisson 
careening  and  settling  out  of  level.  This  danger  was  obviated, 
however,  by  cleaning  out  sections  of  about  10  ft.  square,  one 
at  a  time,  leaving  the  rest  of  the  caisson  well  supported.  The 
rock  at  the  bottom  of  these  pits,  if  sloping,  was  blasted  to  an 
irregular  surface,  forming  depressions  and  elevations.  The 
concreting  was  then  commenced  and  carried  up  to  the  cutting- 
edge  and  packed  under  the  shoulders.  Another  section  was 


3 06  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


then  completed  in  a  similar  manner.  Where  the  depth  to  the 
rock  did  not  exceed  5  or  6  ft.  no  trouble  arose  ;  but  in  greater 
depths  the  material  under  and  outside  the  cutting-edge  would 
cave  in,  endangering  the  safety  of  the  caisson  by  a  sudden 
escape  of  the  compressed  air,  called  “  blow-outs.”  This  would 
frequently  happen  in  sand  and  gravel,  but  seldom  in  clay  or 
silt.  To  avoid  this  difficulty  the  pits  were  lined  with  frames 
and  sheeting,  as  in  sinking  shafts  into  the  ground.  These  tim¬ 
bers  had  to  be  cut  of  the  proper  lengths  and  carried,  one  by 
one,  down  the  shaft;  but  by  this  means  pits  12  to  15  ft.  deep 
were  sunk  and  filled  with  concrete.  In  sand  and  gravel  it  was 
often  impossible  to  hold  the  material,  and  the  framing  or 
bulkheads  would  break  in,  followed  by  much  inflow  of  the 
material  and  escape  of  the  air,  but,  gaining  little  by  little,  the 
entire  side  would  sooner  or  later  be  sealed  up.  This  difficulty 
in  sand  and  gravel  arises  from  the  fact  that  the  pressure  can¬ 
not  be  kept  up  greater  than  that  due  to  the  depth  of  the  cut¬ 
ting-edge  below  the  surface  of  the  water,  as  the  escape  of  the 
air  is  so  great.  In  clay  or  silt  the  material  itself  is  air-tight  or 
nearly  so  at  that  depth,  and  the  pressure  can  be  raised  to 
almost  any  extent.  Caving  in  also  occurred  in  this  material 
to  some  extent  when  unsupported,  but  it  could  be  easily  held  in 
place.  In  the  case  of  the  caisson  to  which  the  accident  hap¬ 
pened,  as  already  described,  we  were  compelled  to  blast  the 
rock  to  a  depth  of  about  7  ft.  around  a  part  of  the  caisson  in 
order  to  level  it.  This  still  left  about  13  ft.  to  be  excavated 
below  the  cutting-edge  at  the  other  parts,  which  was  done  as 
above  described.  Having  in  this  manner  constructed  a  wall  of 
concrete  entirely  around  the  caisson,  the  material  enclosed  was 
then  removed.  Blasts  were  put  in  all  the  sloping  surfaces, 
bringing  the  entire  surfaces  to  a  series  of  depressions  and  rises- 
in  both  directions,  in  order  to  prevent  any  danger  of  sliding. 
No  attempt,  however,  was  made  to  cut  the  rock  to  a  level 
or  even  to  a  series  of  steps,  the  surface  being  simply  very 
much  roughened.  Some  engineers  have  drilled  large  holes  in 
the  rock  and  inserted  iron  rods  projecting  a  foot  or  more  above 
the  rock  in  order  to  prevent  sliding. 


CAISSON  SINKING. 


30  7 


44.  The  filling  of  the  air-chamber  with  concrete  was  then 
proceeded  with.  All  the  concrete  was  mixed  in  batches,  using 
about  a  barrel  of  cement  to  the  batch.  This  was  mixed  by 
hand  on  a  platform  above  and  was  passed  through  the  supply- 
shaft,  which  was  simply  a  long  air-lock  formed  by  a  door  at 
top  and  bottom.  When  the  signal  was  given  the  concrete  was 
mixed  and  immediately  shovelled  into  the  shaft,  the  lower  door 
being  closed  and  further  supported  by  a  timber  strut.  When 
a  sufficient  quantity  had  been  thrown  in — from  |  to  1  cubic 
yard — the  upper  door  was  closed,  the  air  equalized,  and  the 
lower  door  opened,  the  concrete  dropping  on  a  platform.  It 
was  then  carried  in  barrows,  deposited  in  place,  and  rammed. 
Before  throwing  the  concrete  into  the  shaft  several  buckets 
of  water  were  thrown  in,  and  also  after  throwing  the  concrete 
in.  The  water  prevented  the  cement  from  adhering  to  the 
shaft  and  from  heating  and  setting  too  rapidly  when  the  com¬ 
pressed  air  entered  the  shaft ;  otherwise  the  shaft  would  be 
blocked,  and  it  would  be  difficult  to  clear  it  again.  The  hot 
air  of  the  chamber,  unless  a  plenty  of  water  is  used,  causes  the 
cement  to  set  before  it  can  be  properly  handled.  It  requires 
great  care  and  a  concrete  rather  dry  and  mixed  with  very 
small  chips  of  stone  to  pack  close  against  the  deck  of  the 
caisson.  It  is  better  to  leave  one  or  two  sections  of  shaft  in 
place.  The  upper  sections  can  be  removed  and  used  over 
again. 

45.  There  is  nothing  of  special  note  in  the  Schuylkill  River 
caissons  except  their  great  length  as  compared  with  the  width, 
which  was  required  by  the  line  crossing  the  stream  very  ob¬ 
liquely.  The  abutment  caisson  was  nearly  square.  No  crib  was 
used,  but  a  high  coffer-dam  was  constructed  on  the  caisson  :  this 
was  filled  solid  with  rubble  masonry,  one  man  stone  bedded  in 
concrete  ;  the  air  chamber  was  filled  with  concrete.  As  this 
caisson  had  to  support  the  thrust  of  a  heavy  mass  of  earth 
resting  on  the  swamp,  timber  strut  braces  and  large  iron  tie- 
rods  were  used  in  the  working  chamber  to  prevent  sliding,  and 
the  bed  was  given  a  slight  slope  against  the  direction  of  the 
pressure.  (Plate  XVI,  Figs.  1,  2,  and  3.) 


308  a  practical  treatise  on  foundations. 


46.  The  points  especially  worthy  of  notice  in  the  Tombig- 
bee  River  bridge  was  the  nature  of  the  material  on  which  the 
structure  was  built,  and  an  accident  that  happened  to  one  of 
caissons,  from  which  some  useful  information  can  be  obtained. 

The  site  of  the  bridge  was  inaccessible  ;  the  river  itself  being 
the  only  avenue  for  transportation,  and  this  alternating  between 
extreme  high  and  low  water.  All  materials  except  timber  had 
to  be  transported  long  distances.  Brick  and  shells  were  used  in 
the  concrete.  The  material  underlying  the  water  was  a  shifting 
sand,  resting  on  a  silt  intermixed  with  irregular  bowlders,  or 
broken  layers  of  a  bluish-white  marl.  There  was  23  ft.  of  water 
at  the  lowest  stage;  but  'sudden  rises  of  35  to  40  ft.  often  oc¬ 
curred,  and  at  irregular  and  uncertain  periods.  A  simple  crib  or 
open  caisson,  resting  on  the  bed  of  the  river,  would  inevitably 
have  scoured  out ;  nor  could  piles  be  relied  upon,  as  owing  to 
the  irregular  layer  of  marl,  through  which  they  could  not  be 
driven,  some  would  have  scoured  out.  A  coffer-dam  would 
have  been  required  of  great  height,  and  liable  at  any  time  to  be 
scoured  out  or  flooded  ;  and  in  addition,  the  varying  depths 
of  the  borings  left  it  uncertain  as  to  the  proper  depth  at  which 
the  structure  should  be  founded.  For  these  reasons  the  writer 
determined  to  sink  pneumatic  caissons,  as  then  all  doubts  and 
difficulties  could  be  settled  at  the  proper  time.  The  octagonal 
caisson  was  sunk  through  23  feet  of  water,  9  feet  of  sand,  silt, 
and  patches  of  marl,  and  the  excavation  carried  about  9  feet 
below  the  cutting-edge  in  silt.  It  was  found  impracticable  to 
sink  the  caissons  farther,  although  the  entire  air-pressure  was 
let  out  of  the  caisson.  This  indicated  an  unusual  frictional  re¬ 
sistance  on  the  outside ;  doubtless  due  to  the  marl  bowlders 
bearing  strongly  against  the  sides  of  the  caisson.  The  pier, 
however,  at  this  time  was  only  about  one  half  completed  ;  but 
with  this  large  and  well-defined  frictional  resistance,  and  the 
fact  that  borings  indicated  an  almost  unfathomable  depth  of 
silt  below,  it  was  determined  to  build  at  that  point.  The  cais¬ 
son  was  filled  with  concrete  resting  on  the  silt  ;  this  material  was 
so  soft  that  a  rod  four  or  six  feet  long  could  be  readily  pressed 
into  the  material.  The  writer’s  experience  with  driving  piles. 


CAISSON  SINKING. 


309 


and  their  great  bearing  capacity  in  that  kind  of  material,  to¬ 
gether  with  the  fact  that  on  the  same  river  screw-pile  piers 
constructed  by  him,  having  only  100  sq.  ft.  bearing  to  the  pier 
and  carrying  spans  150  ft.  long,  carrying  the  heavy  loads  of  the 
present  day,  had  stood  for  nearly  twenty  years,  gave  confidence, 
as  this  pier  would  have  fully  1000  sq.  feet  of  bearing,  although 
it  would  carry  heavier  piers  and  longer  spans.  The  weight  at 
the  time  the  caisson  was  stopped  was  1,684,500  lbs. ;  and  as  the 
caisson  did  not  rest  on  anything  at  the  bottom,  the  entire  cut¬ 
ting-edge  being  cleared  in  order  to  sink  the  caisson,  if  possible, 
and  the  air-pressure  entirely  relieved,  it  was  a  clear  case  of 
balance  by  friction.  And  as  the  exposed  surface  was  1200  sq. 
ft.  the  frictional  resistance  must  have  been  1400  lbs.  per  square 
foot.  Then  concrete  was  packed  under  the  cutting-edge  and 
shoulders  on  the  lower  side,  and  the  pressure  again  lowered  ; 
the  weight  now  acting  with  a  lever-arm  of  about  10  feet.  The 
frictional  resistance  on  about  one  half  of  the  exposed  surface 
would  be  acting  with  an  arm  of  about  20  feet  or  more.  But 
the  caisson  did  not  settle  a  particle ;  this  seemed  to  be  conclu¬ 
sive  as  to  the  ultimate  bearing  of  the  foundation.  The  com¬ 
pleted  structure,  including  the  rolling  load  on  the  bridge,  weighed 
4,374,500  lbs.  ;  area  of  base  of  caisson,  1148  sq.  feet;  bearing 
resistance  of  foundation-bed,  not  considering  any  allowance  for 
friction  =  3810  per  sq.  ft.;  and  allowing  1,684,500  lbs.  for  fric¬ 
tional  resistance,  the  pressure  on  the  silt  is  2343  lbs.  per  sq.  ft. 
This  is  not  an  unusual  pressure  for  this  material,  as  seen  in 
paragraph  306,  Part  I,  Table  No.  6. 

This  is  particularly  mentioned  as  a  safe  load  at  that  depth, 
on  the  softest  material  that  can  be  called  solid  or  earth.  This 
bridge  has  been  in  use  now  for  over  six  years.  The  piers  were 
built  of  brick,  and  carry  275-ft.  spans.  Such  spans  on  brick 
piers  are  somewhat  unusual.  The  brick  was  hard,  sound,  well 
Burnt,  and  laid  in  cement  mortar. 

47.  One  of  the  rectangular  caissons  45  X  23  ft.  X  14  ft., 
with  a  crib  20  ft.  high,  partly  filled  with  concrete  weighing  800 
tons,  simply  resting  on  the  bottom,  was  lifted  by  the  water  in 
a  rapid  rise  of  the  river ;  and  although  well  secured  to  a  num- 


3io 


A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


ber  of  clusters  of  piles  driven  around  it,  and  swung  askew  of 
its  proper  position  and  dropped  io  ft.  down  stream,  it  could 
not  be  pulled  back  into  position  against  the  current,  and  had 
to  be  flooded  where  it  was.  Sinking  somewhat  suddenly,  the 
material  at  the  upper  end  was  scoured  out;  this  swinging  in¬ 
stantaneously  into  the  eddy  under  and  at  the  down-stream  end 
collected  into  a  mound,  and  when  the  flood  subsided  the  cais¬ 
son  was  found  in  an  inclined  position  at  an  angle  of  about  350 
or  40°.  A  contract  was  made  with  parties  accustomed  to 
lighter  vessels  across  the  bar  below  Mobile  to  lift  the  caisson 
into  position.  The  first  difficulty  was  in  getting  chains  under 
it.  This,  however,  was  ultimately  accomplished,  the  lighters 
lowered,  necessary  connections  made,  the  water  pumped  out ; 
but  the  caisson  did  not  lift,  the  largest  iron  chains  snapping 
and  breaking.  Failing  in  this  the  concrete  was  blasted  out  of 
the  crib  ;  the  caisson  did  not  float  until  air  connections  were 
made  and  air-pressure  put  on,  when  it  rose  suddenly.  It  was 
then  located  and  the  work  proceeded  to  a  finish  as  usual,  but 
many  thousands  of  dollars  had  then  been  wasted. 

The  first  lesson  to  be  learned  from  this  accident  is  that  it  is 
unwise  to  attempt  to  resist  the  action  of  such  rapid  and  high 
rises  in  rivers.  Had  this  crib  been  flooded  in  the  earlier  stages 
of  the  rise,  and  had  we  waited  patiently  for  the  fall  of  the  river, 
both  time  and  money  would  have  been  saved;  and,  second,  it  is 
a  waste  of  time  and  money  to  endeavor  to  lift  such  structures  in 
place.  It  is  far  better  to  lighten  the  load,  and  let  natural  laws 
and  forces  aid  in  the  floating  of  the  structure.* 

From  the  contract  prices  paid  on  this  work,  which  were 
$42.00  per  1000  ft.  B.  M.  of  timber,  $10.00  per  cubic  yard  for 
concrete  in  crib,  and  $15.00  for  concrete  in  caisson,  5  cts.  per  lb. 
for  iron,  and  20  cts.  per  cubic  foot  of  excavation  sinking  cais¬ 
sons,  the  cost  of  the  work  below  water  would  be  $15.06  per 
cubic  yard.  The  actual  cost,  taking  in  consideration  accidents, 
delays,  and  loss  of  material,  was  considerably  greater. 

*  It  is  but  justice  to  say  that  Mr.  H.  F.  Lofland,  the  Div.  Engineer  in  charge 
of  the  bridge,  earnestly  pressed  the  importance  of  flooding  the  caisson  in  time 
to  have  saved  it,  but  unwise  counsels  prevailed,  and  it  was  not  flooded  until  too 
late — a  valuable  but  expensive  experience. 


COMBINED  OPEN-CRIB  AND  PNEUMATIC  CAISSON.  3H 


Article  LI. 

COMBINED  OPEN-CRIB  AND  PNEUMATIC  CAISSON. 

48.  As  was  mentioned,  the  writer  designed  a  combined 
structure  for  the  purpose  of  reaching  rapidly,  economically, 
and  certainly  a  depth  beyond  that  at  which  the  pneumatic 
caisson  can  be  sunk,  upon  which  he  secured  a  patent.  This 
structure  will  now  be  described,  both  on  account  of  its  being  in 
its  general  design  typical  of  both  the  general  construction  of 
a  timber  or  iron  caisson,  and  of  the  novel  features  making  it 
available  for  use  as  a  pneumatic  caisson,  or  an  open  crib. 

The  description  will  be  better  understood  by  referring  to 
Plates  XI  and  XII,  Figs.  1,  2,  3,  and  4.  As  a  crib,  the  descrip¬ 
tion  already  given  will  suffice  (see  paragraphs  2,  3,  4,  Art.  47, 
Part  Third). 

As  a  caisson  it  may  be  stated  in  general  terms  that  there  are 
one  or  more  decks  or  roofs,  converting  that  portion  of  the  crib  be¬ 
low  into  a  caisson  ;  these  roofs  are  removable  in  part  or  entirely. 
As  many  separate  and  distinct  air-locks  as  may  be  desired  or 
required  can  be  introduced.  An  iron  shaft  can  be  extended 
throughout  the  entire  height  of  the  crib  ;  any  part  of  this  shaft 
or  its  entire  length  can  be  converted  into  an  air-lock.  Piles  of 
50  ft.  or  more  can  be  introduced  into  the  air  chamber  and 
driven  below  the  lower  edge  of  the  caisson.  The  general  ad¬ 
vantages  secured  are  that,  1st.  To  the  depth  of  a  hundred  feet, 
or  whatever  may  be  the  limit  of  the  pneumatic  process,  we  se¬ 
cure  the  advantages  attaching  to  this  process.  2d.  That  be¬ 
low  this  depth  the  structure  can  be  used  as  an  open  crib,  sunk 
by  the  usual  methods,  securing  a  minimum  vertical  lift  of  the 
dredged  material, — a  largely  reduced  frictional  resistance  on  the 
outside  surface,  thereby  enabling  greater  depths  to  be  reached 
than  in  any  other  manner  more  rapidly  and  at  less  cost.  And, 
3d,  should  for  any  reason  the  crib  be  stopped  by  any  obstruc¬ 
tion,  long  piles  can  be  introduced  and  driven  until  a  satisfactory 
bearing  is  obtained.  4th.  It  is  specially  applicable  in  very 
great  depths  of  water  where  the  bed  of  the  stream  has  not 


312  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 

bearing  resistance  sufficient  to  build  upon  it,  and  where  the 
excessive  lift  of  the  dredged  material  would  greatly  increase 
the  cost  of  construction.  5th.  It  provides  those  conditions 
and  means  of,  to  a  great  extent,  removing  the  injurious  effects 
resulting  from  working  in  compressed  air,  adding  to  the  com¬ 
fort  and  health  of  the  men,  without  obstructing  or  delaying 
the  prosecution  of  the  work,  and  adding  but  little  to  the  cost 
of  the  structure  itself.  6th.  For  small  depths,  after  sinking  as 
a  pneumatic  caisson,  the  roof  can  be  removed,  after  partly  or 
entirely  filling  the  air-chamber  with  concrete,  by  which  a  solid 
and  uniform  mass  of  concrete  or  masonry  can  be  built  from 
bottom  to  top  of  the  piers. 

49.  Fig.  1,  Plate  XI,  is  a  vertical  cross-section,  showing 
double  walls  of  crib  and  cross-walls  A,  which  are  to  be 
filled  with  gravel  or  concrete,  which  furnishes  the  weight  nec¬ 
essary  to  sink  the  caisson  and  also  forming  a  part  of  the  per¬ 
manent  foundation.  As  seen  in  the  drawing,  V-shaped  cutting- 
edges  are  formed  and  built  solid  for  a  height  of  about  9  ft., 
through  which  both  screw  and  drift  bolts  are  passed,  and  all  of 
the  walls  tied  together  with  long  iron  rods.  From  that  height 
the  several  walls  are  formed  by  12  X  12  in.  timbers  laid  on  top 
of  each  other  and  drift-bolted;  cross-braces  at  intervals  tie  the 
walls  together.  These  walls  are  built  up  as  the  caisson  sinks; 
the  extreme  outside  walls  are  built  with  a  gentle  batter,  or  the 
lower  section  alone  may  have  a  batter,  and  all  above  vertical  ; 
this  latter  is  common,  especially  when  iron  is  used.  The  parti¬ 
tions  C  from  wall  to  wall  constitute  the  various  roofs,  dividing 
the  space  between  the  walls  into  a  number  of  chambers  8  or  9 
ft.  high,  marked  B  in  the  drawing.  The  entire  outside  and 
also  the  roofs  are  calked  or  otherwise  made  air-tight.  Air¬ 
locks  D  built  into  the  roofs  afford  means  of  passing  from 
chamber  to  chamber.  In  the  middle  space  an  iron  shaft  ex¬ 
tends  from  top  to  bottom,  any  part  of  which  or  the  entire  shaft 
can  b.e  converted  into  an  air-lock.  These  shafts  communicate 
by  side-doors  with  the  chambers.  An  air-pressure  due  to  the 
depth  can  be  maintained  in  the  chambers,  the  difference  of 
pressure  in  the  successive  chambers  being  that  due  to  the 
height  of  the  chambers.  Any  number  of  roofs  may  be  used. 


COMBINED  OPEN-CRIB  AND  PNEUMATIC  CAISSON.  3 1 3 

The  roofs  in  the  middle  space  are  constructed  with  iron  beams 
and  plates  riveted  to  them  ;  those  in  the  two  outside  spaces 
are  shown  with  a  timber  construction.  Also  air-locks  D'  afford 
a  communication  from  the  chambers  to  the  spaces  between 
the  walls,  an  open  vertical  shaft  being  left  in  the  concrete  ; 
the  men  having  the  choice  of  entering  or  leaving  the  caisson 
by  this  avenue,  this  twofold  avenue  increasing  the  chances 
of  escape  in  case  of  accidents.  The  usual  air,  water,  and 
discharge  pipes,  P,  are  shown.  The  drawings  show  some  of 
the  air-locks  in  section,  others  in  elevation.  The  doors  are 
shown  both  while  open  and  closed.  Fig.  2  is  a  horizontal  sec¬ 
tion  showing  the  roofs  partly  removed  ;  B,  chambers ;  C,  roofs; 
JD,  air-locks  and  shafts.  As  the  roof  may  be  formed  of  iron 
beams  and  plates,  the  roofs  can  be  opened  by  removing  the 
plates,  leaving  the  girders  to  serve  as  braces;  or,  as  shown  at  T, 
the  plates  under  two  of  the  girders  can  be  left  in  place,  thereby 
forming  troughs  into  which  the  dredged  material  can  be  emp¬ 
tied,  and  discharged  by  the  air  through  pipes.  The  proper 
spaces  are  shown  partly  filled  with  concrete  in  both  drawings. 
Figs.  3  and  4  are  part  sections,  the  first  showing  the  method  of 
introducing  piles  into  the  caisson  through  long  air-locks  and  at 
the  bottom  piles  driven  and  partly  filled  over  with  concrete. 
Fig.  4  shows  the  caisson  sunk  below  the  limit  of  the  pneumatic 
process,  in  which  the  lower  roof  C  has  been  removed  except  as 
to  necessary  bracing;  this  roof  just  passing  below  the  water 
surface,  the  roof  C  is  as  yet  intact. 

Object  and  Uses  of  the  Above  Structure. 

50.  Assuming  a  depth  of  water,  say  100  ft.,  underlaid  by  a 
soft,  silty  material,  into  which  piles  can  be  easily  driven  thereby 
securing  a  sufficient  support.  A  caisson  of  this  kind  could  be 
sunk  resting  on  the  bed  of  the  river.  Piles  could  then  be  in¬ 
troduced  after  the  air-pressure  was  established,  as  shown  in 
Fig.  3,  and  driven  to  the  required  resistance,  cut  off  squared, 
capped  if  desired,  and  then  concrete  built  over  them  to  any  de¬ 
sired  height,  and  the  masonry  then  commenced.  The  ma¬ 
sonry,  if  desired,  could  be  commenced  on  top  of  the  piles. 


314  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 

This  evidently  furnishes  an  economical  mode  of  securing  a 
foundation  where  the  depth  of  the  water  is  great  and  the  un¬ 
derlying  material  uncertain. 

The  crib  resting  on  the  bottom  at,  above,  or  below  the 
limit  of  the  pneumatic  process,  with  the  roof  C'  at  this  level, 
the  roof  C  could  be  partly  removed,  leaving  the  trough-shaped 
braces  in  place  ;  the  material,  dredged  and  lifted  into  this 
trough,  could  be  discharged,  either  by  the  air-pressure  or  mud 
pump,  through  proper  discharge  pipes.  As  the  caisson  sinks, 
the  roof  C  reaching  the  water  surface,  the  roof  Cx  could 
then  be  partly  removed ;  the  men  using  this  as  a  platform 
from  which  to  work.  BB2  then  being  the  work  chamber, 
B passing  below  the  water  surface  gradually.  When  Ct 
reaches  the  limit,  the  men  ascend  to  and  so  on.  These 
operations  are  indicated  in  Fig.  4.  In  this  manner  we  make 
use  of  the  pneumatic  pressure  as  far  as  practicable.  We 
limit  the  lift  of  the  dredged  material  to  a  minimum,  and  se¬ 
cure  the  advantage  of  the  rapid  and  economical  methods  of 
removing  the  material  adopted  in  ordinary  caissons.  The 
water  is  kept  at  a  constant  level,  the  men  ascending  as  the 
caisson  sinks.  In  addition,  the  air  escaping  under  the  cutting- 
edges  and  rising  along  the  sides  reduces  materially  the  resist¬ 
ance  of  friction  by  loosening  the  material.  There  can  be 
no  doubt  that  this  process  is  reliable,  expeditious,  and  economi¬ 
cal,  and  can  be  used  where  other  means  would  fail.  If  the  depth 
should  be  200  ft.  below  the  water  surface,  say  70  feet  water 
and  130  ft.  solid  material,  sink  the  caisson  by  the  pneumatic 
process  100  ft.  At  this  point  the  dredging  would  commence, 
the  lift  gradually  increasing  from  o  to  100  ft.,  or  an  average 
lift  of  50  ft.,  the  air  or  the  pump  doing  the  balance.  In  the 
open-crib  process  the  dredging  would  commence,  when  the 
caisson  rested  on  the  bed  of  the  river,  the  first  height  of  lift 
being  70  ft.,  gradually  increasing  to  200  ft.,  the  average  lift 
being  135  ft.  It  is  perfectly  evident  that  this  method  must  be 
slower,  more  expensive,  and  more  uncertain. 

51.  The  construction  is  by  no  means  a  bad  one  for  a  cais¬ 
son  to  be  sunk  less  than  100  ft.,  or  for  an  ordinary  caisson. 


COMBINED  OPEN-CRIB  AND  PNEUMATIC  CAISSON.  3  IS 

It  would  not  require,  before  commencing  to  sink  the  caisson,, 
the  delay  necessarily  caused  by  the  time  required  to  construct 
the  ordinary  caisson  proper.  The  heavy  mass  of  timber  re¬ 
quired  in  the  roof  would,  to  a  large  extent,  be  avoided.  Only 
one  roof  would  be  necessary  in  this  case;  but  it  would  be  ad¬ 
visable  to  use  at  least  two,  the  chamber  between  being  used 
for  a  dressing  and  warming  room  for  the  caisson  men,  through 
which  they  could  pass  as  leisurely  and  as  comfortably  as  may 
be  desired,  without  obstructing  in  any  manner  the  progress  of 
the  work.  It  is  evident  that  the  roofs  should  be  of  iron,  as  it 
can  be  more  conveniently  constructed  and  removed.  It  has 
the  further  advantage  that,  in  case  it  should  be  found  neces¬ 
sary  after  sinking  the  caisson  to  go  beyond  the  pneumatic 
limit,  additional  roofs  could  be  constructed  and  the  sinking 
continued,  or  piles  introduced  and  driven ;  whereas,  in  the 
pneumatic  caisson  proper,  when  its  limit  is  reached,  it  can 
neither  be  sunk  further  nor  removed  ;  and  it  is  possible  that, 
under  such  circumstances,  the  structure  would  be  useless,  its 
entire  cost  thrown  away,  or  an  uncertain  foundation  used. 

52.  The  entire  structure  can  be  built  either  of  iron  or  wood, 
the  choice  being  mainly  one  of  cost,  as  the  strength  in  either 
case  is  sufficient,  or  the  part  below  the  bed  of  the  river  could 
be  wood  and  the  part  above  of  iron — especially  if  in  sea  water, 
where  the  timber  would  be  destroyed  by  worms,  and  also 
where  obstruction  to  the  current  or  navigation  is  a  matter  of 
moment,  somewhat  less  space  would  be  occupied  by  the  iron 
wall.  In  short,  the  writer  does  not  hesitate  to  say  that  it  is  a 
good  design  for  any  kind  of  foundation  below  water  for  any 
depth  of  water  or  solid  material  from  30  to  200  ft.  It  has, 
however,  its  special  application  in  those  rivers,  such  as  the 
Mississippi,  at  New  Orleans,  where  there  is  a  great  depth  of 
water,  and  where  any  such  obstruction  to  the  channel  would 
be  bitterly  opposed,  as  the  structure  could  be  narrowed  to  a 
minimum  thickness  at  any  desired  depth  below  the  water  sur¬ 
face,  without  in  any  manner  interfering  with  the  prosecution 
of  the  work  below.  “  It  is  claimed  in  the  patent  that  greater 
depths  can  be  reached  than  by  any  other  known  method,  and 


316  a  practical  treatise  on  foundations. 

at  any  depth  the  work  can  be  done  relatively  more  rapidly, 
more  economically,  and  more  certainly,  and  that  for  such 
depths  as  require  only  the  ordinary  coffer-dam  absolute  se¬ 
curity  against  breaks  and  leaks  can  be  secured,  and  founda¬ 
tions  can  be  constructed  either  under  a  moderate  pressure,  or 
after  fully  bracing  and  sealing  up  the  cutting-edge  the  roof 
can  be  removed,  and  the  work  proceeded  with  safely  and  se¬ 
curely,  as  in  an  ordinary  coffer-dam  in  the  open  air.”  See 
Plates  XI  and  XII. 

In  the  year  1889  the  writer  showed  Mr.  E.  L.  Corthell,  an  en¬ 
gineer  of  great  ability  and  experience,  the  plans  and  descriptions. 
He  was  then  working  on  the  Memphis  Bridge  Plans.  It  had 
been  supposed  by  many  that  the  depths  required  for  that  bridge 
would  be  much  greater  than  were  afterward  found  necessary. 

In  1890  Mr.  Corthell  made  a  report  on  building  a  bridge 
across  the  Mississippi,  near  New  Orleans,  and  in  this  report  he 
recommended  the  above  plan. 

GENERAL  REMARKS. 

53.  In  all  of  the  foregoing  subjects  the  writer  has  de¬ 
scribed,  in  general  terms,  the  actual  methods  of  the  construction 
of  caissons,  cribs,  and  coffer-dams,  etc.,  as  practised  by  himself 
and  many  other  engineers,  and  also  the  subsequent  operations 
of  sinking,  with  more  or  less  detail,  without  criticism  of  the 
methods  of  others.  He  has,  however,  often  alluded  to  the 
importance  of  avoiding,  as  far  as  practicable,  the  adoption  of 
what  seemed  to  be  useless  refinement  in  the  sizes  and  quan¬ 
tities  of  materials  used  in  such  structures,  as  well  as  in  the 
manner  of  putting  the  parts  together,  necessitating  increased 
cost  and  time  required  in  construction.  And  in  all  designs  his 
aim  has  been  to  keep  in  view  that  good  engineering  practice 
only  requires  that  all  structures  should  be  constructed  in  the 
least  possible  time,  and  the  least  possible  cost,  consistent  with 
strength,  durability,  permanency,  and  suitableness  to  the  end  in 
view.  That  this  does  not  seem  to  be  the  practice  of  many 
engineers  is  apparent  in  many  structures  and  in  many  portions 
of  the  same  structure,  and  as  they  do  not  generally  result  in 


COMBINED  OPEN-CRIB  AND  PNEUMATIC  CAISSON.  3 1 7 

any  better  work  and  only  add  to  the  time  and  cost,  such  prac¬ 
tice  can  only  be  considered  useless  and  wasteful  of  both  time 
and  money.  Attention  to  some  extent  was  called  to  this  sub¬ 
ject  in  discussing  the  subjects  of  concrete  and  masonry,  and 
the  effort  was  there  made  to  show  in  what  manner  first-class 
work  in  every  respect  could  be  secured  without  useless  and 
onerous  requirements  such  as  are  often  imposed.  Attention 
will  now  be  directed  to  similar  requirements  often  imposed  in 
the  construction  of  some  deep  and  difficult  foundations. 

It  is  not  uncommon  to  see  described  in  books  for  the  con¬ 
struction  of  the  sides  of  open  caissons,  which  are  simply  timber 
coffer-dams,  that  they  should  be  composed  of  planed  and 
tongue  and  grooved  timbers,  sometimes  of  specially  large 
cross-sections,  where  as  timber  as  it  comes  from  the  mill  is  in 
every  respect  as  good,  no  planing  being  necessary,  except  pos¬ 
sibly  planing  slightly  the  edges  of  the  outside  plank  for  a 
calking  joint. 

The  guide-piles  of  coffer-dams  are  often  required  to  be 
sawed  square.  Round  piles  are  equally  as  good,  cost  less,  and 
can  be  driven  much  more  satisfactorily.  In  framing  cribs  that 
are  to  be  filled  with  concrete,  it  is  far  better  to  use  round  logs 
for  the  cross  braces,  any  slight  variation  in  the  diameter  at 
the  two  ends  being  a  matter  of  little  or  no  moment,  and 
they  admit  packing  under  and  around  them  to  much  greater 
advantage.  And  in  many  cases  the  entire  crib  could  be  con¬ 
structed  of  round  logs  without  in  any  way  impairing  the  use¬ 
fulness  of  the  structure,  as  for  many  purposes  under  water  sap 
wood  is  as  serviceable  as  heart  wood. 

In  the  construction  of  the  pneumatic  caisson  particularly 
there  seems  to  be  no  regard  paid,  as  a  rule,  either  as  to  the 
cost  or  to  the  relative  strength  of  the  parts ;  bolts  and  rods  are 
inserted  if  large  quantities  where  there  would  seem  to  be  little 
or  no  use  for  them  ;  and  no  special  attention  is  given  to  a  strong 
and  rigid  connection  between  the  walls  of  the  working  cham¬ 
ber  and  deck  of  the  caisson,  which  is  matter  of  the  greatest 
importance.  For  instance,  in  the  deck  of  the  caisson  com¬ 
posed  of  eight  or  ten  courses  of  timber  crossing  each  other, 


3 1 8  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 

drift-bolts  I  in.  X  22  ins.  are  driven  at  every  intersection ;  this 
would  require  in  any  ordinary-sized  caisson  some  32,000  drift- 
bolts,  or  about  190,000  lbs.,  costing  some  $8000  to  $10,000, 
when  one  fourth  to  one  fifth  of  these  quantities  would  be  ample 
under  any  circumstances  ;  and  in  addition  long  bolts  1^  to  2  ins. 
in  diameter  and  8  or  10  ft.  long  are  put  through  the  entire  deck 
with  a  reckless  profusion,  and  only  to  hold  timbers  together 
that  have  little  tendency  to  separate  ;  and  similarly  in  other 
parts,  except  that  comparatively  few  bolts  are  used  to  connect 
the  deck  to  the  walls  of  the  air-chamber,  where  the  danger 
really  exists,  and  where  the  framing  is  usually  such  that,  out¬ 
side  of  the  interior  bracing,  the  bolts  are  the  only  connections. 
Often,  also,  in  constructing  caissons,  all  of  the  timbers  are  run 
through  a  planer,  so  as  to  gauge  them  to  exactly  the  same  size. 
Surely  nothing  is  gained  by  this ;  the  cost,  however,  is  greatly 
increased.  Unless  the  timber  is  badly  sawed,  an  equally  good, 
if  not  better,  work  is  secured  by  bedding  the  timber  in  cement 
mortar,  and  filling  the  vertical  joints  with  grout.  In  regard  to 
incasing  the  cutting  edge  of  a  caisson  in  iron  plating,  there  is 
much  difference  of  opinion  and  practice.  It  can  safely  be  said 
that  it  is  not  necessary;  it  may  be  a  safe  precaution,  and  it 
may  or  may  not  add  materially  to  the  cost.  It  is  claimed  by 
some  engineers  as  a  decided  disadvantage.  The  writer  has 
never  used  it. 

Often  expensive  stagings  and  platforms  are  erected  to 
regulate  and  control  the  sinking  of  caissons:  here  again  the 
writer  cannot  speak  from  experience ;  they  will  certainly  be 
very  costly,  and  their  utility  is  certainly  doubtful.  The  writer 
only  used  a  few  clusters  of  piles,  mainly  to  hold  the  caisson 
while  floating,  and  to  aid  in  locating  the  caisson  accurately  on 
the  bottom,  no  material  error  in  position  resulting  during  the 
sinking.  Tendencies  to  move  gradually  in  one  direction  are 
sometimes  developed,  which  can  generally  be  checked  either 
by  blowing  the  material  to  that  side,  or  by  settling  the  caisson 
slightly  out  of  level,  and  then  levelling  it  again  ;  reasonable  care 
and  watchfulness  will  ordinarily  prevent  any  trouble.  Many 
such  matters  do  not,  of  course,  admit  of  any  close  calculation, 


COMBINED  OPEN-CRIB  AND  PNEUMATIC  CAISSON.  319 

and  for  this  reason  it  is  the  aim  to  be  always  on  the  safe  side, 
which  is  commendable  so  far  as  it  applies  ;  but  there  is  nothing 
gained  by  enormously  strengthening  some  parts  of  a  structure 
and  leaving  other  parts  proportionately  weak.  The  writer’s 
object  is  only  to  call  attention  to  some  of  the  evidently  useless 
waste  of  material,  money,  and  time,  without  any  reasonably 
compensating  advantages.  Spare  no  time  or  money  in  strength¬ 
ening  weak  points,  but  do  not  waste  them  on  those  essentially 
strong  points,  that  can  take  care  of  themselves. 

The  writer  made  estimates  for  contractors  proposing  to 
build  a  large  caisson  and  sink  the  same  under  specifications 
which  fully  illustrates  the  above  remarks.  A  few  extracts  will 
be  given  for  the  corner-posts  : 

4  pieces  of  white  oak,  24  ins.  x  24  ins.  X  16  feet. 

4  “  “  “  16  ins.  X  27  ins.  X  16  feet. 

312  “  “  “  12  ins.  x  16  ins.  X  16  feet. 

The  other  timbers  for  the  caisson  were  of  almost  all  con¬ 

ceivable  dimensions :  12  in.  X  12  in.  X  20  feet,  12  in.  X  12  in.X 
10  ft.  2  in.,  8  in.  X  12  in.  X  17  feet,  and  so  on ;  in  all  908,616  ft. 
B.  M.,  every  stick  of  which  had  to  be  planed.  The  writer  does 
not  hesitate  to  assert  that  an  equally  strong,  durable,  and 
rigid  structure  could  have  been  built  with  no  variations  in  di¬ 
mensions  from  12  ins.  X  12  ins.,  except  in  the  lengths  of  certain 
parts  which  have  necessarily  to  be  specified  ;  and  further,  that 
the  planing  of  the  timbers  was  absolutely  without  necessity 
or  even  advantage.  Pine  timbers  would  have  been  equally 
suitable.  Such  requirements  simply  mean  an  enormous  waste 
of  money  and  time.  In  addition,  screw-bolts  amounting  to 
43,000  lbs.,  in  all  lengths  from  2  to  12J  ft.,  and  from  1  to  2\ 
ins.  diameter,  were  stuck  in  all  conceivable  places  through  the 
deck  of  the  caissons,  through  corner-posts,  etc.,  and  in  addition 
1992  drift-bolts  l|-  in.  diameter  and  58,348  drift-bolts  1  in.  diam¬ 
eter,  amounting  in  the  aggregate  to  350,623  lbs.;  these  were 
used  at  every  intersection  in  the  deck,  that  is,  one  foot  apart 
in  each  direction  over  each  of  the  seven  crosses  of  solid  timber 
in  the  deck.  Whereas  one  fifth  of  the  entire  number  of  bolts 


320  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


would  have  been  ample,  and  the  long  screw-bolts  2\  in.  diame¬ 
ter,  and  1 2  J  ft.  long,  as  well  as  a  number  of  the  other  sizes,  could 
have  been  entirely  omitted,  with  a  saving  of  thousands  of  dol¬ 
lars.  The  writer  suggested  these  changes  to  the  chief  engineer, 
stating  the  useless  labor  and  expense  involved,  only  to  receive 
the  reply  that  the  work  was  to  be  done  rigidly  according  to 
specifications,  and  that  the  company  could  pay  for  it.  The  re¬ 
sult  was  that  the  lowest  bid  was  over  $230,000,  whereas  with 
reasonable  requirements  the  work  could  have  been  done  under 
$200,000.  All  bids  were  rejected  ;  the  company  undertook  the 
work.  Whether  changes  were  made  or  whether  the  cost  was 
more  or  less  is  unknown.  This  is  but  a  sample  of  the  reckless 
waste  of  money  in  designing  and  constructing  many  works  that 
have  come  under  the  writer’s  observation,  and  is  introduced 
to  show  the  importance  of  designing  structures  with  some  re¬ 
gard  to  the  relative  strength  of  the  parts  connected  and  the 
connections  themselves. 

It  will  be  noticed  that  in  this  structure  there  is  380  lbs.  of 
iron  to  every  1000  ft.  B.  M.  of  timber.  In  the  caissons  of  the 
Susquehanna  River  Bridge  the  average  iron  in  bolts  in  each 
caisson  ranged  from  136  to  152  lbs.  per  1000  ft.  B.  M.,  being 
probably  more  in  proportion  on  the  smaller  caissons,  as  many 
straps,  bolts,  etc.,  were  of  the  same  dimensions  in  all  cases ; 
this  proved  ample  in  sinking  through  both  sand  and  gravel 
and  silt,  and. in  one  caisson  a  sudden  sinking  7  or  more  feet 
and  landing  hard  on  rock,  crushing  off  the  lower  end  of  the 
verticals  and  careening  at  a  considerable  angle  with  a  heavy 
load  on  top,  did  not  spring  a  leak  in  the  timber-work  at  any 
point.  On  the  Cairo  Bridge,  from  the  data  before  the  writer, 
the  iron  is  414  lbs.  per  1000  ft.  B.  M.  This  evidently  includes 
the  shafts,  pipes,  etc.,  as  the  amount  of  iron  is  merely  given 
as  so  much  weight  supported  by  the  foundation-bed,  and  as 
both  the  roof  of  the  caisson  and  the  high  cribs  (34  ft.)  were 
open-built,  there  was  relatively  a  smaller  proportion  of  timbers 
and  a  larger  proportion  of  concrete,  necessitating  a  larger  ratio 
between  the  iron  and  timbers,  though  the  actual  quantity  of 
iron  in  pounds  was  small. 


ALL-IRON  PIERS. 


321 


Article  LI  I. 

ALL-IRON  PIERS. 

54.  Iron  piers  can  be  constructed  with  either  cast  or 
wrought  iron  columns.  The  wrought-iron  columns  are  com¬ 
posed  of  latticed  channels ;  several  such  columns  being  placed 
in  slightly  inclined  positions,  these  are  braced  with  horizontal 
channels  or  other  form  of  struts,  and  diagonal  tension  members 
between  them.  In  rivers  liable  to  great  rises,  bringing  large 
masses  of  driftwood,  these  piers  should  be  incased  in  plate- 
iron  ;  this  is  generally  open  work,  consisting  of  flat  strips  placed 
at  intervals,  or  large  lattice  strips  ;  this  while  not  entirely  op¬ 
posing  the  current  turns  aside  the  drift  and  prevents  large 
masses  collecting  or  getting  tangled  up  in  the  braces.  Such 
piers  are  light  and  should  be  strongly  anchored  to  low  masonry 
piers,  the  piers  being  built  up  to  or  a  few  feet  above  low-water, 
and  in  very  high  bridges  up  to  or  above  high-water.  Two 
good  examples  of  such  piers  can  be  seen, — one  across  the 
Alabama  River,  near  Montgomery,  and  a  second  in  a  bridge 
recently  constructed  across  the  James  River,  near  Richmond, 
Va.  There  are  some  serious  objections  to  such  piers,  unless  the 
masonry  is  carried  up  above  high-water,  in  rivers  carrying 
much  drift  or  large  masses  of  ice,  and  they  are  not  common  in 
such  cases. 

55.  A  description  of  an  all-iron  screw-pile  pier  bridge,  con¬ 
structed  by  the  writer  across  the  Mobile  River,  about  16  miles 
above  Mobile,  Ala.,  will  be  interesting  and  instructive.  The 
total  width  of  the  Mobile  River  at  this  point  was  about  1000  ft. 
This  distance  was  divided  into  seven  spans  by  six  screw-pile 
piers  and  two  brick  abutments  resting  on  ordinary  piles.  There 
was  one  draw-span  260  ft.  from  end  to  end,  giving  two  clear 
openings  of  about  112  ft.  each. 

It  may  be  as  well  to  mention  that  screw-piles  may  be  of 
wood  or  iron,  solid  or  hollow,  varying  in  diameter  from  6  to  12 
ins.  or  more,  having  a  screw-disk  at  one  end,  similar  to  one 


322  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


turn  of  an  auger,  which  may  be  from  12  ins.  to  6  ft.  in  diame¬ 
ter.  They  are  screwed  into  the  soil,  soft  rock,  coral  reef,  etc. 
Hand  or  steam  power  can  be  used.  For  ordinary  piers  there 
are  from  6  to  8  piles  to  the  pier.  The  bearing  surface  being 
the  sum  of  the  areas  of  the  screw-disk,  the  friction  of  the 
material  on  the  surface  of  the  shaft  will  add  something  to  their 
bearing  capacity. 

56.  In  each  of  the  screw-pile  piers  in  the  Mobile  River 
there  were  8  solid  wrought-iron  shafts,  diameter  of  screw-disks 
4  ft.  The  pivot  pier  was  composed  of  one  centre  shaft  8  ins. 
diameter,  and  10  shafts  of  6  in.  diameter  distributed  around 
the  circumference  of  a  circle  about  25  ft.  diameter.  The  screw- 
disk  for  the  centre  shaft  was  6  ft.  diameter.  All  other  shafts 
were  6  in.  diameter  with  cast-iron  screw-disks  4  ft.  diameter. 
The  rectangular  piers  were  formed  in  two  rows,  4  piles  to  each 
row.  The  piles  in  each  row  were  8  ft.  centres  ;  the  rows  them¬ 
selves  were  9  ft.  centres.  The  piles  were  braced  by  eye-beam 
struts  connected  around  the  shafts  by  collars  which  were 
bolted  to  the  beams,  and  diagonal  tension  rods  in  both  vertical 
and  horizontal  planes.  The  piles  were  capped  with  heavy 
cast-iron  pieces  bolted  together  through  flanges,  short  wrought 
beams  resting  lengthwise  of  the  pier  on  the  caps,  and  on  these 
a  thick  iron  bridge-seat.  All  parts  well  bolted  together.  The 
eye-beam  struts  with  the  horizontal  diagonal  rods  were  called 
girt  frames.  Three  or  more  of  these  were  used  according 
to  the  height  of  the  pier  above  the  bed  of  the  river.  Drawings 
and  full  details  of  these  piers  are  shown  in  Plate  XXI,  Figs. 
1,  2,  and  3. 

57.  The  piles  of  the  pivot  pier  were  braced  by  eye-beams 
between  the  piles  and  radiating  from  the  centre,  and  a  system 
of  diagonal  tension  rods  in  various  inclinations.  These  piles 
were  also  capped  and  connected  together  on  top  by  large  cast- 
iron  pieces  bolted  together,  upon  which  was  placed  the  neces¬ 
sary  turn  table  arrangements. 

The  general  lengths  of  the  spans  were  about  142  ft.  The 
design  of  the  superstructure  was  the  well-known  but  little  used 
post  truss,  in  which  both  the  tension  and  strut  web  members 


ALL-IRON  PIERS. 


323 


are  inclined.  The  original  contract  contemplated  screwing  the 
piles  to  a  depth  of  45  ft.  below  the  bed  of  the  river ;  the  actual 
result  was  that  the  greatest  penetration  below  the  bed  of  the 
river  was  18^  ft.,  and  the  average  15^  ft.  and  then  in  one  or 
more  cases  the  shaft  commenced  to  twist,  and  in  all  cases  the 
steel  teeth,  presently  to  be  explained,  cut  iron  shavings  from 
the  pile,  without  turning  or  screwing  them  into  the  material, 
which  was  a  fine  compact  sand. 

These  indications  were  accepted  as  satisfactory  proof  that 
sufficient  bearing  power  had  been  secured.  One  idea  in  adopt¬ 
ing  this  kind  of  pier  was  to  prevent  scouring  by  offering  as 
little  obstruction  as  practicable  to  the  current,  and  thereby 
prevent  any  scouring.  These  piers  are  light  but  strong  and 
stiff,  and  have  now  been  in  use  over  twenty  years,  and  carry 
safely  the  heavy  rolling  loads  of  the  present  day. 

58.  Some  little  detail  concerning  the  manner  of  sinking 
them  will  be  given.  The  depth  of  water  varied  from  10  to  20 
feet ;  the  height  of  pier  above  water,  about  9  feet.  At  the 
site  of  the  bridge  the  rise  from  floods  was  only  a  few  feet ;  the 
immense  volumes  of  water  from  the  rivers  above  dividing 
among  many  bayous,  and  spreading  over  the  entire  swamp. 

The  shafts  were  rolled  in  sections  of  different  lengths  ;  the 
bottom  section,  which  was  connected  with  the  screw-disks  by 
four  steel  pins,  was  about  22  feet  long.  This,  when  set  in  place, 
would  reach  above  the  water ;  on  top  of  this  a  heavy  cast-iron 
sleeve  about  3  feet  long,  fitting  snugly  around  the  pile,  and  fast¬ 
ened  to  it  by  two  steel  pins  at  right  angles.  Another  section 
12  or  15  feet  long  was  then  lowered  into  the  sleeve,  and  resting 
on  the  top  of  the  first  shaft.  Steel  pins  were  then  passed 
through  sleeve  and  shaft.  The  machine  for  turning  the  piles 
consisted  of  a  rectangular  base  frame  of  timber,  to  the  corners 
of  which  were  fastened  four  stout  pieces  of  timber  meeting 
at  a  point  above,  which  was  slightly  out  of  centre  in  one 
direction,  so  that  the  shaft  when  standing  vertical  and  in 
position  would  clear  the  timbers  of  the  frame.  About  four 
feet  from  the  bottom  of  the  frame  a  large  cog-wheel,  sup¬ 
ported  horizontally,  was  placed ;  the  spokes  of  this  wheel 


324  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


rested  in  an  iron  collar  about  15  ins.  diameter,  carrying  on 
the  inside  two  friction-rollers.  The  shaft  passed  through 
the  centre  of  the  collar.  A  strong  jacket  with  flanges  in 
two  halves,  and  carrying  on  the  interior  several  solid  steel 
plugs  with  sharp  teeth,  was  adjusted  to  the  shaft  and  drawn 
by  bolts  close  to  it,  indenting  the  teeth  into  it  by  bolts 
through  the  jacket-flanges,  the  lower  ends  of  which  rested 
against  the  friction-rollers ;  a  worm-screw  with  ordinary  crank- 
arms  could  be  thrown  in  or  out  of  gear  with  the  cog-wheel.  A 
number  of  men  turning  the  worm  by  its  arms  imparted  a  circular 
motion  to  the  cog-wheel,  which  turned  the  jacket  and  the  em¬ 
braced  shaft,  thereby  screwing  the  disks  into  the  bed  of  the 
river.  This  could  be  continued  until  the  top  of  the  jacket 
reached  the  rollers ;  the  jacket  was  then  loosened  and  lifted  to 
a  distance  equal  to  its  length,  again  tightened  to  the  shaft, 
when  the  power  could  again  be  applied.  From  8  to  10  men 
could  apply  power  enough  to  twist  the  shaft.  The  use  of 
steam  might  have  been  more  economical  and  rapid,  but  would 
not  have  been  more  efficient.  The  greatest  difficulty  existed 
in  starting  and  holding  each  pile  vertical  and  in  its  exact  posi¬ 
tion  ;  this  was  essential,  as  otherwise  the  caps  and  girt-frames 
could  not  have  been  adjusted  to  the  piles,  as  all  parts  were 
made  in  Chicago  and  shipped  to  the  bridge-site.  The  work 
was  carefully  and  conscientiously  executed  by  Gen.  Win.  Sooy 
Smith.  In  addition,  every  pile  had  to  be  brought  to  exactly 
the  same  level  on  top,  as  it  would  have  been  very  troublesome 
and  expensive  to  cut  them  to  a  level.  This  was  accomplished 
in  the  case  of  every  pile,  except  the  large  centre-pile  of  the 
pivot  pier,  which  could  only  be  screwed  9  feet  into  the  sand ; 
this  was  clipped  off  with  thq  cold-chisel.  The  greatest  error  in 
the  levels  of  the  top  did  not  exceed  one  eight  of  an  inch. 

When  from  any  cause  it  is  necessary  to  reach  greater 
depths  than  the  piles  can  be  screwed  by  turning,  the  limit  of 
which  is  reached  when  the  piles  show  signs  of  twisting,  or  the 
teeth  or  other  hold  upon  the  pile  is  insufficient,  resort  can  be 
had  to  the  water-jet.  This  method  has  been  used  successfully. 
It  is  stated,  no  doubt,  upon  reliable  authority  that  the  use  of 


LOCATION  OF  PIERS. 


325 


the  jet  is  more  effective  when  applied  to  the  upper  surface  of 
the  screw-disc,  rather  than,  as  would  seem  natural,  to  the  under 
side  and  the  point  of  the  pile.  Why  so,  does  not  seem  entirely 
clear.  Applied  to  both  under  and  upper  surfaces  would,  no 
doubt,  be  advantageous.  In  this  process  there  would  seem  to  be 
no  cause  of  trouble  when  the  screw-disc  was  not  over  from  12  to 
18  inches  in  diameter;  but  with  discs  from  3  to  6  feet  in  diam¬ 
eter  it  would  be  troublesome  to  hold  a  pile  exactly  vertical  and 
in  exact  position,  if  this  should  be  absolutely  necessary,  as  in 
the  case  already  mentioned.  On  this  point  the  writer,  how¬ 
ever,  cannot  express  an  opinion,  as  he  has  no  experience  in  this 
method  of  sinking  screw-piles.  Each  pile  in  each  pier  had  to 
be  located  separately  from  an  established  base  on  the  shore 
or  from  completed  piers,  as  no  staging  could  have  been  con¬ 
structed  steady  enough  to  maintain  any  centre  point. 

Article  LIII. 

LOCATION  OF  PIERS. 

59.  There  are  many  methods  of  locating  the  piers  of  large 
bridges  across  rivers.  They  all,  however,  resolve  themselves 
into  the  method  of  triangulation,  or  direct  measurement  from 
some  established  base  on  the  shore  ;  and  as  it  all  depends  then 
on  the  base-line,  this  should  be  accurately  measured,  and  its 
direction  and  location  in  regard  to  the  centre-line  of  the  bridge 
should  be  carefully  selected.  It  should  be  as  nearly  at  right 
angles  to  the  centre-line  as  practicable ;  and  its  length  should 
be  equal,  or  nearly  equal,  to  the  entire  width  of  the  river,  so 
that  distances  from  the  end  of  the  base,  equal  to  that  of  each 
pier  from  the  same  point,  can  be  laid  out  on  the  base-line.  It 
is,  however,  rare  that  both  of  these  conditions  can  be  realized 
in  practice  ;  especially  as  it  is  also  desirable  that  the  base-line 
should  be  laid  out  on  ground  as  nearly  level  as  practicable.  This, 
however,  is  not  a  matter  of  so  much  importance,  as  with  due 
care  perfectly  accurate  distances  can  be  measured  on  rolling 
or  rough  ground.  But  it  is  essential  that  each  pier  shall  be 


3 26  A  PRACTICAL  TREA 


TISE  ON  FOUNDATIONS. 


easily  visible  from  its  own  triangulation  points,  and  that  the 
entire  base  shall  be  seen  from  either  end.  The  best  adjustment 
of  the  base  to  all  of  these  conditions  must  be  made.  No  angle 
in  a  triangle  should  be  less  than  30  degrees,  nor  greater  than 
120  degrees.  The  base  may  be  somewhat  less  in  length  than 
the  width  of  the  river.  It  is  advisable  to  have  a  base 
on  both  sides  of  the  river,  the  one  used  as  a  check  — T~l 


on  the  other;  2d.  If  points  can  be  found,  two  on 
each  side  of  the  river,  so  that  the  lines  joining  two 
of  them  is  near  to,  and  approximately  parallel  to, 
the  centre-line  of  the  bridge,  and  so  situated  that 
each  and  every  pier  can  be  seen  from  both  extremi¬ 
ties  of  each  line,  these  lines  form  an  excellent  basis, 
and  are  good  checks  on  each  other.  The  lengths 
of  these  lines  have  to  be  determined  from  bases, 
which  form  well-conditioned  triangles  with  them ; 
but  otherwise  selected  without  reference  to  the  centre-line 
of  the  bridge  or  the  positions  of  the  piers.  If  the  two  sec¬ 
ondary  bases  across  the  river  are  in  sight  of  each  other  the  one 
can  be  used  to  calculate  the  length  of  the  other,  thus  insuring 
the  accuracy  of  both.  These  lines  should  be  far  enough  from 
the  centre-line  so  that  the  directions  of  each  pier  from  its  ex¬ 
tremities  shall  form  well-conditioned  triangles  with  the  base ; 
3d.  Or  bases  can  be  measured  on  opposite  sides  of  the  river, 
extending  in  opposite  directions— one  up  and  one  down  stream.’ 
Upon  these  lines  points  can  be  established,  so  that  the  lines 
joining  two  of  the  points  shall  intersect  the  centre-line  at  the 
centre  of  each  pier.  This  method  has  the  advantage  that  when 
these  points  are  once  accurately  located  it  is  not  necessary  to 
turn  any  angles  to  locate  the  position  of  a  pier,  as  it  depends 
upon  the  intersection  of  two  lines  ranged  by  foresights,  and  the 
further  advantage  that  the  engineers  are  working  from  largers 
to  smallers,  and  any  error  in  centring  the  rods  with  the  tran¬ 
sits  are  divided  or  lessened,  eliminating  two  sources  of  error  : 
that  of  working  from  smallers  to  largers,  if  the  one  base  is  too 
short,  and  the  error  of  graduation  in  the  limb  of  the  transit,  as 
well  as  the  error  in  reading  the  vernier.  In  addition,  this 


LOCATION  OF  PIERS. 


3  27 


method  only  requires  the  measurement  of  one  angle  for  each 
base,  viz.,  the  angle  between  the  base-lines  and  the  centre-line 
of  the  bridge;  the  base-lines  need  not  be  parallel.  The  dis¬ 
tances  from  ends  of  the  base-line  to  each  pier,  measured  on  the 
centre-line,  must  be  known.  Having  determined  the  width  of 
the  river  between  points  established  on  the  shore  on  the  centre¬ 
line,  and  the  position  of  the  piers  on  this  line,  the  piers  can  be 
located  by  either  of  the  three  methods.  Figs.  9,  10,  and  11  show 

Fig.  9.  Fig.  io. 


these  methods  in  their  order  as  above  described — in  which  AB 
is  the  centre-line  ;  BC,  the  base-lines,  from  which  the  piers  are  to 
be  located  ;  1,  2,  3,  4,  and  5,  the  positions  of  the  piers.  The 
base-lines  in  Figs.  9  and  10  are  shown  as  passing  through  the 
centres  of  the  shore  piers,  and  the  triangulation  points  at  the 
centres  of  the  piers  on  the  other  side  of  the  river.  These  may 
occupy  any  position  with  respect  to  the  piers  that  may  be 
found  most  convenient.  In  Fig.  9  the  angle  at  B  is  known,  and 
also  the  distances  BG,  BH,  BK,  and  B2,  i?3,i>4,  and  ^5,  and  the 


328  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


angles  at  G,  H,  and  K  calculated.  In  Fig.  n  the  bases  B2C 
B,C,  are  calculated  from  the  measured  bases  B,,  D,,  and  DC 
respectively,  the  accuracy  of  which  can  be  tested  by  calculating 
B,C,  from  B.2C  used  as  a  base.  The  distance  from  I  and  5 
piers  can  then  be  calculated  from  either  B2C  or  B,C,  as  a  base 
— these  piers  being  located  on  the  banks  of  the  stream,  as  may 
be  determined  by  purely  practical  considerations;  the  main 
object  being  to  place  them  far  enough  back  from  the  sloping 
banks  to  preclude  any  danger  from  caving  in  of  the  banks. 
The  angles  B^iC  and  C2B„  are  then  easily  calculated  in  the  tri¬ 
angle  iZ>22,  AB2,  1.2,  and  the  angle  Z?2 1.2  are  known,  from  which 
\B.,2  can  be  calculated ;  then  2BJA  —  i.B^C —  i.A22,  and  similarly 
for  all  other  required  angles.  As  seen  DC  and  B,D,  can  be  meas¬ 
ured  where  convenient  without  reference  to  the  centre-line  AB. 
In  Fig.  10,  having  measured  the  base  BC,  lay  off  the  distances 
BK,  BH,  BG,  and  BC,  approximately  equal  to  B4,  A3,  B2,  and 
B\,  respectively.  The  proper  distances  AG,,  AH,,  and  AK, 
can  be  easily  calculated.  For  instance,  we  know  in  the  triangle 
B2G,  BG,  B2,  and  the  angle  GB2,  then  angle  G2B  can  be 
calculated.  Then  in  the  triangle  A2G,  we  know  the  angle  A2G, 
=  G2B,  the  angle  2 AG, ,  and  the  distance  A2,  from  which  cal¬ 
culate  AG,.  In  every  case  two  transits  are  required  to  locate 
the  position  of  the  piers ;  one  of  them  at  least  should  be  of  first- 
class  make,  with  a  good  telescope  and  accurate  limb  gradua¬ 
tions.  Several  points  in  the  prolongation  of  AB  on  each  side 
of  the  river  should  be  established  ;  and  large  hubs  of  good  solid 
wood,  from  2\  to  3  feet  long,  should  mark  these  points — the 
exact  point  marked  by  a  tack.  The  top  of  these  hubs  should 
be  even  with  the  surface  of  the  ground,  or  better,  a  few  inches 
below,  to  prevent  its  being  disturbed  by  hauling  over  or  near  it. 
The  centre-line  should  be  well  and  distinctly  marked  on  the 
faces  of  the  piers  before  they  rise  above  the  line  of  site.  The 
intersection  of  all  the  oblique  lines,  with  the  faces  of  piers, 
should  be  marked  also.  A  line  of  red  or  black  paint  answers 
for  this  purpose ;  and  on  the  completion  of  the  pier  its  exact 
centre,  both  as  to  distance  and  line,  should  be  marked  by  two 
chisel-scratches  intersecting  and  painted,  or  by  drilling  a  small 


LOCATION  OF  PIERS. 


329 


hole  and  inserting  a  short  iron  rod.  In  measuring  the  base-lines 
large  hubs  should  be  driven  not  over  12  to  15  feet  apart,  accu¬ 
rately  lined,  on  level  ground  ;  these  should  then  be  sawed  off 
square  to  the  same  level.  On  rolling  ground  as  many  should  be 
cut  off  to  the  same  level  as  practicable ;  and  any  change  in  the 
level  required  is  to  be  made  at  one  point,  and  then  cut  off  as 
many  as  practicable  at  the  new  level.  The  base  can  then  be 
measured  with  an  accurate  steel  tape  ;  driving  tacks  in  line,  and 
at  the  proper  distances  apart,  to  mark  the  important  points. 
The  most  satisfactory  method  is  to  have  made  at  least  three 
timber  base-bars,  12  to  15  feet  long;  these  are  made  of  two 
pieces  of  white  pine  about  i|-  in.  thick  and  3  in.  wide;  the  one 
set  edgewise  on,  and  at  right  angles  to,  the  other,  and  bolted 
together,  showing  a  T-section.  These  can  be  lightened  by 
rounding  gently  from  the  centre  to  the  ends.  Brass  strips  with 
pyramidal-shaped  ends  are  then  bolted  to  their  ends.  Having 
obtained  a  3-ft.  standard  U.  S.  steel  bar,  these  bars  are  accu¬ 
rately  measured  by  them,  the  brass  point  being  filed  to  some 
exact  distance,  say  15  feet — the  brass  point  not  being  over  ^  in. 
square  ;  the  measurement  being  made  at  the  standard  tempera¬ 
ture  as  nearly  as  practicable  ;  the  sketch  (see  Fig.  8)  shows  one 
end  of  the  base-bar  ready  for  use.  These  three  bars  should  then 
be  placed  in  line,  resting  on  top  of  the  hubs,  with  their  brass 
points  in  contact  ;  the  rear  one  should  then  be  moved  to  the 
front  and  placed  in  contact  with  the  front  one,  and  so  on  ;  the 
extreme  front  end  being  marked  on  the  tack  with  a  scratch 
to  avoid  slight  errors  caused  by  moving  the  rods.  This  should 
be  continued  from  end  to  end  of  base,  and  repeated  several 
times ;  then  checked  by  steel  tape-measure.  In  a  number 
of  bridges  across  wide  rivers,  with  high  piers  placed  at  all 
intervals  from  100  to  525  feet  apart,  the  writer  has  never 
had  any  appreciable  errors  in  locating  the  piers. 

60.  He  has,  however,  relied  to  a  great  extent  on  measure¬ 
ments  with  steel  wire,  using  generally  what  is  known  as  No. 
10  pianoforte  wire.  This  is  very  strong  and  light,  can  be 
pulled  almost  to  a  horizontal  line  with  a  spring-balance  ;  a  pull 
of  15  to  20  lbs.  is  sufficient.  The  base-line  was  measured 


330  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 

carefully,  as  already  described  ;  after  which  all  hubs,  except 
those  marking  the  lengths  of  the  span,  should  be  removed,  so 
that  in  stretching  the  wire  it  will  have  the  same  sag  that 
it  would  have  when  locating  the  piers.  These  hubs  were  well 
protected,  so  that  they  could  not  be  disturbed.  A  tarred 
string  will  adhere  to  the  wire  when  tightly  wrapped  around 
over  a  distance  of  i  to  ins.,  the  inner  edges  of  the  string 
being  at  the  required  distance  apart  from  tack  to  tack  on  the 
base-line.  This  distance  being  measured  with  wooden  rods  of 
standard  lengths,  is  independent  of  the  temperature.  The  wire 
should  be  stretched  on  the  base  before  measuring  the  span.  The 
contraction  or  expansion  can  be  allowed  for  on  the  spring, 
when  appreciable,  without  moving  the  strings  ;  and  after 
measuring  the  span  it  should  again  be  tested  on  the  base. 
This  is  a  safe  precaution,  but  the  tarred  string  never  slipped 
in  the  writer’s  experience.  A  change  of  temperature  of  i8o° 
would  change  the  length  of  a  wire  525  ft.  long  0.66  of  a  foot, 
— about  8  inches,  coefficient  of  expansion  taken  at  .00125. 
Assuming  that  the  strings  were  adjusted  at  a  temperature  of 
6o°,  then  at  a  temperature  of  90  0  the  length  would  be  al¬ 
tered  1^-  ins.,  that  is,  lengthened  ;  and  at  30°  it  would  be 
shortened  1^  ins.  These  would  be  nearly  the  extreme  ranges 
of  temperature.  But  this  is  of  no  moment,  as  the  wire  is 
tested  on  the  base  before  measuring.  A  little  greater  or  less 
pull  on  the  spring-balance  would  correct  the  error.  Both  the 
transit  and  wire  should  be  used  to  check  each  other.  The 
transit-rod  for  this  work  should  be  a  f  or  £  in.  pipe,  brought  to 
a  well-defined  point  at  one  end,  and  painted  in  alternate 
lengths  of  a  foot  red  and  white. 

LOCATION  OF  BRIDGES. 

61.  The  writer  has  been  often  asked  what  are  the  consid¬ 
erations  determining  the  location  of  bridges.  The  factors  enter¬ 
ing  into  this  matter  are  various.  1st.  Economy;  this  involving 
such  questions  as  the  width  of  the  river,  the  depth  of  the  water, 
the  nature  of  the  material  forming  the  bed  of  the  river,  the  depth 
of  the  foundation-bed  below  the  surface,  etc.;  the  slowness  or 


LOCATION  OF  BRIDGES. 


331 


rapidity  of  the  current.  These  questions  must  all  be  consid¬ 
ered  and  that  site  selected  which  costs  the  least,  if  economy 
alone  is  to  be  considered.  High  banks  on  one  or  both  sides 
are  generally  desired,  as  they  decrease  the  cost  of  the  ap¬ 
proaches,  though  they  may  increase  the  cost  of  the  bridge 
proper.  Again,  without  regard  to  cost  of  the  bridge  proper, 
the  necessary  or  best  location  of  the  line  on  either  side  of  the 
bridge  may  be  the  controlling  consideration.  This  may  or  may 
not  be  controlled  by  a  question  of  total  cost.  A  good  illus¬ 
tration  of  this  is  in  the  case  of  the  Susquehanna  River  bridge 
at  Havre  de  Grace.  If  the  line  had  been  located  two  miles 
higher  up  the  river,  a  bridge  could  have  been  constructed  rest¬ 
ing  on  solid  rock  exposed  at  low-water,  instead  of  building  a 
bridge  at  a  point  where  we  had  to  go  to  a  depth  of  90  ft. 
below  water  surface  for  a  foundation-bed.  This  would  have, 
however,  lengthened  the  line  some  four  or  six  miles,  and  would 
have  caused  some  sharp  curves.  Six  miles  extra  distance 
causes  much  extra  cost,  both  in  construction  and  in  mainte¬ 
nance  for  all  time,  and  means  ten  or  fifteen  minutes’  more 
time  in  running  between  Baltimore  and  Philadelphia. 

62.  Bridges  should  be  easily  approached  from  both  direc¬ 
tions,  avoiding  both  sharp  curves  and  steep  grades.  In  fact, 
we  are  often  forced  to  build  at  certain  points,  no  choice  being 
left  to  the  engineers,  especially  in  crossing  navigable  rivers,  as 
permission  has  to  be  obtained  from  the  Secretary  of  War,  and, 
in  addition,  he  determines  the  lengths  of  the  spans,  heights  of 
the  piers,  as  well  as  site  of  bridge.  The  necessities  of  the  case 
first  determine  the  site.  After  this  economy,  considered  as  ap¬ 
plied  both  to  the  bridge  and  the  construction  of  the  line  on 
both  sides,  determines  the  selection  of  a  bridge  site. 

63.  Economy  also  demands  the  height  of  the  piers  to  be 
as  little  above  high-water  as  practicable.  On  navigable 
streams  this  height  is  regulated  ordinarily  by  law,  whether  a 
draw-span  is  used  or  not.  Likewise,  to  a  large  extent,  the 
position  of  piers,  as  well  as  length  of  span,  is  determined  by 
law.  But  when  not  so  regulated,  it  may  be  stated  as  a  gen¬ 
eral  rule  that,  where  the  foundations  are  inexpensive,  rela- 


332  A  PR  A  CTICAL  TREATISE  ON  FOUNDATIONS. 


tively  speaking,  a  number  of  piers  and  short  spans  will  be 
economical.  Where  the  foundations  are  deep  and  costly,  few 
piers  and  long  spans  are  to  be  preferred.  In  either  case  the 
aim  should  be  to  make  the  total  cost  as  small  as  possible  by 
many  trials  with  different  lengths  of  span. 

64.  It  is  advisable,  as  far  as  possible,  to  avoid  bends  in  the 
river,  as  the  piers  should  always  be  placed  with  their  longer 
axis  parallel  to  the  current  ;  for  the  same  reason,  the  line 
should  cross  the  stream  at  right  angles  to  the  direction  of  the 
current. 

65.  The  following  table  gives  a  few  examples  of  the  longest 
bridges,  longest  single  spans,  with  the  highest  piers  and  lowest 
foundation-beds  now  in  existence: 


Total 

Longest 

Nature  of  , 

- —Depth  Sunk - . 

Length. 

Span. 

Foundation.  Low-water.  High-water. 

New  York  Suspension  Bridge  5890 

1595 

Caisson 

78 

Poughkeepsie  Cantilever . 

4595 

548 

Crib 

132 

Havre  de  Grace  Truss . 

6300 

525 

Caisson 

90 

94 

Memphis  Cantilever . 

7997 

790 

*  ‘ 

96 

131 

Hawkesbury  Truss . 

416 

Crib 

153 

160 

The  Forth  Cantilever,  2  spans, 

each . 

I7IO 

St.  Louis  Steel  Arch . 

1550 

520 

Caisson 

94 

136 

The  above  data  are  taken, 

in  some  cases, 

,  from 

unofficial 

sources,  but  are  very  close  approximations,  and  serve  the  pur¬ 
pose  of  showing  the  depths  which  can  be  reached  by  well- 
known  methods  of  construction. 

* 

Article  LIV. 

THE  POETSCH  FREEZING  PROCESS. 

66.  The  writer  will  give  a  short  description  of  the  freezing 
process,  which  has  been  used  to  a  limited  extent  in  sinking 
very  deep  shafts,  and  generally  through  the  most  difficult  and 
treacherous  material  with  which  the  engineer  has  to  deal, 
namely,  quicksand,  which  always  is  troublesome  and  expen¬ 
sive  to  encounter,  and  often  has  opposed  an  insurmountable 
barrier  to  further  progress.  It  has  been  used  in  Europe  to  a 
considerable  extent,  but  to  a  very  limited  extent  in  this  coun- 


THE  FOETSCH  FREEZING  PROCESS. 


333 


try.  It  has  been  successful  where  applied,  but  the  public  are 
as  yet  to  a  great  extent  left  in  ignorance  of  its  relative  cost, 
nor  has  its  possibilities  been  sufficiently  developed  to  form  a 
definite  opinion  as  to  its  range  of  applicability.  The  owners 
of  this  patent  are  the  well-known  and  reliable  firm  of  Sooy- 
smith  &  Co.,  and  it  will  doubtless  be  pressed  to  its  full  practi¬ 
cable  value  and  usefulness  by  them.  The  following  brief  de¬ 
scription  is  obtained  from  them  and  other  sources : 

67.  A  series  of  vertical  pipes  10  ins.  in  diameter,  open  at 
both  ends,  are  sunk  around  the  space  to  be  excavated  to  rock 
or  some  impervious  strata.  These  may  be  called  the  pilot 
pipes.  Inside  of  these,  pipes  8  ins.  diameter,  tightly  closed  at 
the  lower  ends.  Inside  of  these  latter  pipes,  smaller  pipes, 
open  at  the  bottom,  are  inserted.  Each  set  of  pipes,  being 
connected  in  a  series  by  itself,  communicates  either  directly  or 
indirectly  with  a  cooling  tank.  The  freezing  liquid  is  pumped 
through  the  inner  small  pipes  and  returns  through  the  outer 
larger  pipes  to  the  cooling  tank,  to  be  cooled  again  and  again 
circulated  through  the  pipes.  For  convenience  and  economy 
these  pipes  are  arranged  in  a  circular  form  around  the  space  to 
be  excavated.  As  the  cooling  mixture  circulates  it  freezes  the 
soil  in  the  form  of  an  increasing  solid  cylinder  or  core,  which 
unites  at  points  between  the  pipes,  thus  forming  a  solid  frozen 
wall  around  the  space,  the  enclosed  space  being  either  entirely 
or  partly  frozen.  The  excavation  is  then  commenced,  leaving 
sufficient  thickness  of  frozen  wall  to  resist  the  outside  pressure. 
For  safety,  however,  in  the  present  development  of  the  process 
the  shafts  are,  or  have  been,  lined  with  frames  and  sheeting  as 
the  excavation  progressed.  The  costs  then  are  :  first,  the  cost 
of  the  necessary  machinery  and  plants,  including  pipes  and  the 
freezing  fluids,  etc. ;  second,  the  sinking  of  the  pipes  to  the 
required  depths,  and  subsequent  removal  of  the  same  ;  third, 
the  excavation  of  the  material  in  its  frozen  state.  This  last 
must  necessarily  be  very  expensive,  as  it  is  estimated  that  the 
crushing  resistance  of  frozen  quicksand  may  be  as  high  as  1000 
lbs.  per  square  inch.  Lining  may  not  be  necessary  when  the 
frozen  wall  is  cylindrical,  with  small  diameters  ;  but  with  large 


334  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 

rectangular  piers  they  would  have  to  be  of  very  great  thickness 
to  resist  the  outside  pressure,  unless  well  braced  against  it. 
Experience  will,  however,  settle  these  points,  and  speculation 
is  of  but  little  profit.  Such  is  the  process,  and  very  simple 
it  is. 

68.  The  conductivity  of  earthy  materials  either  partly  or 
fully  saturated  with  water  is  not  known,  and  as  there  is  doubt¬ 
less  more  or  less  movement  of  the  water  in  the  water-bearing 
strata,  a  sufficient  degree  of  cold  must  be  provided  and  kept 
up  during  the  entire  time  of  excavating  and  lining  the  shaft. 

It  is  estimated  that,  as  the  specific  heat  of  quicksand  is 
only  one  fifth  as  much  as  that  of  water,  the  amount  of  cold 
necessary  to  freeze  I  cu.  yd.  of  water  would  freeze  2-|  cu.  yd. 
of  quicksand,  and  that  one  horse-power  per  day  would  freeze 
362  lbs.  of  water. 

69.  Ordinary  refrigerating  machines  act  upon  the  principle 
that  when  a  gas  is  compressed  its  temperature  rises  and  when 
it  expands  its  temperature  falls.  Ammonia,  having  a  high 
specific  heat,  is  probably  the  most  economical  gas  to  use. 
“  The  ammonia  may  be  compressed  mechanically,  or  it  may  be 
compressed  by  the  tension  of  its  own  vapor  heated  in  a  still,” 
which  is  cooled  by  passing  through  coils  of  pipe  immersed  in 
water,  retaining  its  pressure  in  the  still,  and  when  allowed  to 
expand  in  other  coils  or  pipes  its  temperature  falls  rapidly  to 
well  below  zero.  In  this  condition  it  absorbs  heat  from  any- 
material  with  which  it  comes  in  contact,  by  which  its  own  tem¬ 
perature  rises.  It  is  then  cooled,  allowed  again  to  expand, 
with  the  ultimate  result  of  freezing  the  earth  or  water  sur¬ 
rounding  the  pipes.  The  efficiency  of  the  now  existing  ma¬ 
chinery  is  only  about  twenty-five  per  cent  of  the  energy 
applied.  The  cold  gas  may  be  circulated  directly  through  the 
pipes  in  contact  with  the  soil,  or  it  may  more  conveniently  be 
employed  to  cool  a  brine,  which  is  then  circulated  through  the 
pipes.  At  Iron  Mountain,  Mich.,  where  a  shaft  15  ft.  square  was 
sunk  to.  the  depth  of  100  ft.,  there  were  used  twenty-seven  pipes 
8  ins.  diameter,  arranged  on  a  circumference  of  29  ft.  in  diameter, 
the  pipes  being  a  little  over  3  ft.  apart.  In  ten  days  from  start- 


THE  POETSCH  FREEZING  PROCESS. 


335 


•V  T3« 

PCETSCH-SOOYSMITH  FREEZING  CO 

Na  2  Nassau  Street.  New  York. 


Fig.  i2. 


33^  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


ing  the  frozen  cylinders  were  in  contact.  From  this  time  the 
enclosed  space  froze  more  rapidly  than  outside  the  pipes,  for 
obvious  reasons.  Strata  containing  little  water  were  frozen  to 
a  greater  distance  from  the  pipes  than  those  containing  much 
water.  “An  ammonia  machine  of  the  compression  type  (the 
ammonia  compressed  mechanically)  was  used.  Its  capacity 
was  twenty-five  tons  of  ice,  or  fifty  tons  refrigerating  capacity, 
per  day.  The  wall  was  frozen  and  the  excavation  to  the  ledge 
of  rock  (ioo  ft.  down)  was  completed  in  two  and  a  half  months 
from  the  time  that  the  ice  machine  first  started.”  The  cir¬ 
culating  brine  was  calcium  chloride,  on  account  of  its  low 
freezing-point,  high  specific  heat,  and  non-corroding  action  on 
iron  pipes.  “  The  best  results  are  obtained  from  such  a  rate 
of  circulation  that  there  is  but  little  difference  in  temperature 
between  the  outgoing  and  incoming  brine.  A  very  efficient 
temperature  for  the  outgoing  brine  is  io°  F.  below  zero,  and 
pumped  at  such  a  rate  that  the  return  flow  is  2°  higher.”  The 
subject  is  a  very  interesting  one,  and  it  remains  as  yet  to  be 
determined,  the  cost  as  compared  with  other  methods  at  the 
same  depth,  its  certainty  as  against  leaks,  breaking  in  of  walls 
at  great  depths,  the  relative  time  taken  to  complete  structures 
requiring  such  large  bases  as  the  piers  of  bridges;  and,  until 
applied  on  such  large  scales  which  may  develop  either  unknown 
difficulties  or  advantages  in  the  process,  it  would  be  unjust  to 
the  owners  and  to  the  engineering  profession  alike  to  forebode 
either  evil  or  good  concerning  it,  and  it  is  to  be  hoped  that  its 
owners  may  be  bold  enough  to  make  the  experiment  on  a  large 
scale.  The  drawing,  Fig.  12,  page  335,  shows  positions  and 
arrangement  of  pipes,  the  excavation  made  through  sand, 
gravel  and  bowlders  to  rock,  and  the  timber  lining  for  shaft. 

Article  LV. 

QUICKSAND. 

70.  HAVING  now  described  the  various  materials  on  which 
structures  are  more  usually  built,  and  the  many  means  adopted 
to  secure  a  safe  bearing  for  both  shallow  and  deep  foundations, 
a  few  facts  in  connection  with  the  nature  of,  and  difficulties 


QUICKSAND. 


337 


to  be  encountered  in  dealing  with,  the  most  troublesome, 
treacherous,  and  almost  unmanageable  material,  namely, 
quicksand,  will  be  interesting  and  instructive. 

It  is  not  uncommon  to  consider  as  quicksand  any  kind  of 
material,  so  saturated  with  water,  that  it  will  flow  more  or  less 
freely  when  its  natural  condition  of  equilibrium  is  destroyed,  by 
excavating  pits,  trenches,  shafts,  or  tunnels.  This  material  may 
be  found  on  the  surface  underlaid  by  a  firm  material,  or  it  may 
be  found  in  strata  of  greater  or  less  thicknesses  confined  by  firm 
strata  both  above  and  below.  When  on  the  surface,  though 
presenting  some  difficulties,  it  can  be  dealt  with  by  any  of  the 
methods  heretofore  described,  and  will  not  therefore  be  further 
discussed  here.  The  most  troublesome  case  arises  when  strata 
of  quicksand  are  met  with  at  considerable  depths  below  the 
surface.  The  reasons  are  many  and  evident :  the  pressure  is 
likely  to  be  much  greater;  the  flow  of  the  material  allows  the 
superincumbent  strata  to  settle,  bringing  an  almost  irresistible 
pressure  upon  the  sides  of  the  structure,  either  crushing  it  in 
or  at  any  rate  throwing  it  out  of  line,  increasing  greatly  the 
amount  of  material  to  be  excavated  ;  these  causes  adding 
enormously  to  the  cost  of  the  structure  and  time  required  to 
complete  it.  Sometimes  the  crude  methods  of  overcoming  these 
difficulties,  regardless  of  delay  and  cost,  such  as  the  free  use  of 
straw,  brush,  shavings,  extra  sheeting  and  bracing,  have  proved 
successful,  but  often,  after  repeated  efforts,  and  expenditure  of 
money,  the  further  prosecution  of  the  work  had  to  be  aban¬ 
doned.  The  discovery  and  application  of  the  freezing  process 
was  a  source  of  hope  and  encouragement,  and  that  it  is  effec¬ 
tive  cannot  now  be  questioned  or  denied.  This  and  the  intro¬ 
duction  of  a  new  method,  presently  to  be  explained,  requires  a 
more  accurate  understanding  and  definition  of  the  loose  ma¬ 
terial  called  quicksand. 

71.  In  a  pamphlet  written  by  Mr.  E.  L.  Abbott,  dated 
Nov.  20,  1889,  on  the  freezing  process,  and  doubtless  having 
the  sanction  of  such  high  authority  as  the  Sooysmith  Com¬ 
pany,  quicksand  is  defined  as  any  earth  which  “  will  in  some 


33 8  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


degree  run  like  a  fluid  when  mixed  with  water.”  He,  however, 
states  that  any  kind  of  sand  mixed  with  a  small  amount  of 
clay  possesses  this  property,  but  that  the  most  troublesome 
material  contains  but  a  small  per  cent  of  very  fine  sand.  “  This 
material  when  undisturbed  may  have  some  consistency  ”  (italics 
mine)  ;  when  disturbed  will  flow  through  any  minute  opening. 
In  the  Engineering  News,  April  28,  1892,  in  which  the  new  in¬ 
vention  of  Mr.  R.  L.  Harris  is  described,  is  found  this  state¬ 
ment:  “This  quicksand,  when  dry,  is  an  impalpable  powder. 
When  saturated  with  water  it  is  very  compact  and  hard  until 
disturbed.  Under  the  pressure  of  a  slight  depth  it  becomes 
apparently  almost  solid  ;  hammer  strokes  of  300  ft.-lbs.  aided 
by  a  wash  pipe,  causing  2-in.  iron  pipes  to  penetrate  upon  an 
average  less  than  0.1  in.  per  blow.  Upon  being  agitated  with 
water  the  quicksand  becomes  alive  and  runs  like  mush.  Its 
currents  under  pressure  move  glacier-like,  and  are  seemingly 
irresistible.” 

The  writer  built  some  culverts  on  a  quicksand  ;  the  solid 
material  composing  it  was  an  impalpable  powder,  and  it  would 
run  into  the  excavation  like  “mush.”  He  also  drove  piles  for 
a  trestle  several  hundred  feet  in  length  through  what  was 
called  quicksand.  No  difficulty  occurred  so  far  as  penetration 
was  concerned,  the  piles  moving  several  feet  at  a  blow,  but 
immediately  after  impact  the  piles  would  lift  the  hammer,  and 
removing  it,  they  would  spring  up  suddenly  to  the  height  of 
several  feet.  There  is  clearly  several  kinds  of  this  troublesome 
material. 

72.  The  freezing  process  is  applicable  whether  the  solid 
material  composing  the  quicksand  is  clay,  sand,  or  mixed  clay 
and  sand.  Time  and  expense  alone  are  questions  to  be  con¬ 
sidered.  This  method  has  been  explained. 

73.  The  latest  method  (see  Engineering  News')  is  novel;  ex¬ 
periment  proves  it  effective  in  the  material  described.  The 
importance  of  knowing  the  nature  of  the  solid  material  arises 
from  the  method  adopted,  as  it  depends  upon  the  hardening 
of  injected  cement.  It  is  generally  accepted  that  pure  cement 


QUICKSAND. 


339 


mixed  with  clay  or  mud  or  exceedingly  fine  sand  *  either  does 
not  harden  at  all,  or  at  any  rate  imperfectly ;  and  in  such  cases 
the  cement  must  be  “  doctored  ”  with  sand,  plaster  of  Paris,  or 
anything  else  that  will  solidify  under  the  existing  conditions. 
In  the  case  to  be  described,  the  work  consisted  in  thus  solidify¬ 
ing  a  trench  for  a  large  intercepting  sewer,  the  laying  of  which 
had  baffled  all  the  efforts  of  the  engineers,  and  practically 
bankrupted  the  contractors. 

When  such  high  authority  as  th t  Engineering  News  pub¬ 
lishes  the  following  :  “  This  [the  freezing]  process  has  proved 
very  useful  in  many  cases,  but  from  its  very  nature  it  requires 
a  somewhat  expensive  refrigerating  plant,  a  long-continued 
circulation  of  the  freezing  fluid,  and  a  continuance  of  the  cir¬ 
culation  so  long  as  it  is  desired  to  keep  the  material  solid,  if  it 
is  to  be  exposed  for  any  considerable  time.” 

“  The  method  to  be  described  is  the  invention  of  Mr. 
Robert  L.  Harris,  N.  Y.  Am.  Soc.  C.E.,  and  it  has  been  re¬ 
cently  tested  experimentally  on  a  sufficiently  large  scale  to 
establish  fully  its  practicability,  under  proper  conditions,  in  our 
judgment.  It  seems  likely  to  prove  a  competitor  of  the 
freezing  process  in  some  fields,  besides  having  useful  applica¬ 
tion  in  cases  where  that  process  would  not  be  suitable.” 

The  writer  needs  no  excuse  for  giving  a  detailed  descrip¬ 
tion  of  the  method. 

The  principle  involved  is  simple,  and  depends  only  upon  the 
fluidity  of  the  material  when  mixed  with  water.  If  two  pipes 
be  sunk  into  the  material  at  distances  apart  varying  with  the 
depth,  and  a  current  of  water  be  forced  through  one  of  them, 
it  will  seek  an  outward  passage  along  the  line  of  least  resistance, 
and  will  issue,  carrying  some  of  the  material  from  the  other  pipe 
along  with  it,  washing  a  channel  between  the  two  pipes,  and 

*  The  writer  condemned  a  large  quantity  of  exceedingly  fine  sand;  subse¬ 
quent  tests  and  experiments  satisfied  him  that  the  mortar  produced  was  equal 
to  any  previously  used  on  the  same  work  with  a  good-sized  and  sharp-grain  pit- 
sand.  Two  large  piers  were  built  of  this  fine  sand  which  stood  immersions  in 
flood  water,  covering  it  shortly  after  being  mixed  and  used  (the  same  day),  it 
also  stood  well  a  severe  winter  on  exposed  surfaces  of  masonry.  It  was  an 
almost  impalpable  powder  when  dry. 


340 


A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


by  using  a  number  of  pipes  a  chamber  will  be  scoured  out. 
Channels  or  chambers  being  made,  “  the  plan  was  then  to  sub¬ 
stitute  for  the  channel-making  stream  of  water  ”  some  cement¬ 
ing  material  in  a  fluid  condition,  and  by  proper  arrangement  of 
valves  shut  the  outlet  pipes  as  the  cementing  fluid  reached 
them,  and  by  applying  pressure  not  only  make  the  cementing 
fluid  fill  the  chamber,  but  also  permeate  the  adjacent  materials, 
thereby  forminga  floor  between  the  pipes  and  by  gradually  raising 
the  pipes  additional  layers  would  be  formed  uniting  into  a  solid 
wall.  The  trench  for  the  sewer  was  12  to  16  ft.  wide  and  from 
20  to  30  ft.  deep  in  quicksand  ;  the  length  through  this  material 
was  over  4000  ft.  An  attempt  to  pump  out  this  material  was 
made,  and  an  area  of  forest  about  150  X  75  ft.  settled  several 
feet,  inclining  large  trees  at  a  considerable  angle  and  doing 
other  damage.  He  found  that  quicksand  in  solution  took 
hours  to  settle,  but  on  introducing  the  cementing  material  all 
solid  matter  settled  rapidly,  leaving  clear  water  on  top;  hence, 
by  agitating  the  material,  a  large  quantity  would  be  placed 
in  suspension,  and  the  introduction  of  the  cementing  ma¬ 
terial  would  result  in  an  intimate  mixture  and  precipitation  of 
the  material,  ultimately  forming  a  solid  floor.  Pipes  had  been 
lowered  to  a  depth  of  25  ft.  and  at  distances  of  4,  10,  and  14  ft. 
apart,  which  established  fully  the  circulating  theory.  Then  4 
pipes  were  sunk  at  the  angles  of  a  quadrilateral  4  ft.  on  the 
side,  and  sunk  17  ft.  below  the  excavated  surface.  A  chamber 
vas  formed,  and  after  maintaining  the  cavity  for  three  days, 
the  cementing  material  was  forced  through  the  pipes  into  the 
chamber,  resulting  in  a  fairly  solid  and  complete  floor.  Two- 
inch  pipes  were  first  sunk,  and  a  small  cavity  hollowed  out  at 
die  bottom.  Smaller  pipes,  carrying  suitable  valves  at  the 
bottom  for  closing  the  pipes  against  upward  currents,  were 
lowered  through  the  larger  ones.  When  the  ends  of  the  smaller 
pipes  were  below  those  of  the  larger,  the  circulation  was 
unobstructed  ;  by  slightly  raising  the  smaller  pipe,  the  fluid 
could  not  escape.  The  blocks  of  cemented  quicksand  were 
from  3  to  6  ins.  thick,  reaching  from  pipe  to  pipe  ;  they  were 
hard  and  solid,  and  homogeneously  solid— in  some  cases  for  a 


QUICKSAND. 


341 


thickness  of  6  ins.  or  more.  The  following  figures  illustrate 
the  process:  Fig.  13  shows  the  manner  of  making  a  solid  wall 
by  successive  slight  lifts  of  alternate  pipes,  using  first  one  and 
then  the  other  for  the  downward  current,  and  the  two  adja¬ 
cent  pipes  for  the  upward  current.  In  forming  a  floor  (see 
Fig.  14),  the  pipes  are  simply  distributed  over  the  space  at 
regular  intervals,  and  sunk  practically  to  the  same  depth  ;  the 
shaded  portions  representing  the  solid  layers  or  blocks  of 


PLAN. 


Fig.  13.  Fig  14. 

Cementing,  through  Pipes,  Quicksand  for  Foundations  or  Walls. 

cemented  quicksand.  Great  claims  are  made  for  this  process 
in  protecting  shores,  driving  shafts,  and  tunnels,  and  in  putting 
in  foundations  forming  the  base  of  the  materials  in  place. 

The  essential  principles  are  to  so  arrange  the  pipes  as  to 
allow  free  circulation  while  washing  out,  and  to  close  the  dis¬ 
charge  pipes  when  the  cementing  material  is  forced  in.  Pipes 
should  not  be  allowed  to  be  caught  in  the  hardened  material. 
Whatever  may  be  the  possibilities  of  the  method,  which  must 
be  established  on  a  sufficiently  large  scale  by  experiment,  the 
process  is  simple,  and  seems  to  be  effective.  Outside  of  the 
pipes  the  only  plant  required  is  a  pump  of  sufficient  capacity 


342 


A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


to  produce  a  good  current  in  a  2-inch  pipe,  and  a  moderate 
pressure  when  introducing  the  cementing  material. 

Both  of  these  methods  are  patented. 

74-  I”  case  of  the  culverts  founded  on  quicksand,  the  writer 
simply  spread  the  base  by  logs  crossing  each  other  in  several 
course,  and  covered  with  plank  upon  which  the  masonry  rested. 
Although  this  method  is  not  to  be  recommended,  and  should 
not  be  used  at  all  under  very  heavy  structures,  the  culverts  have, 
nevertheless,  carried  safely  railway  trains  for  years.  Particular 
care  was  taken  to  sheet  around  the  sides  and  ends  so  as  to 
confine  the  quicksand  as  much  as  practicable. 

75.  Hollow  brick  or  concrete  or  iron  cylinders  and  timber- 
lined  shafts  are  often  sunk  to  great  depths  through  these  soft 
materials,  and  ultimately  filled  with  concrete  or  masonry  col¬ 
umns  or  pillars  The  sinking  is  effected  by  simply  excavating 
the  material  from  the  inside  and  adding  weights,  if  necessary,  to 
the  cylinders  sufficient  to  make  them  sink  against  the  friction; 
or  by  the  ordinary  method  of  suspending  the  upper  top  setting 
or  frame  at  the  surface,  and  as  the  excavation  advances  plac¬ 
ing  other  strong  frames  of  timber  at  intervals  of  4  or  5  feet, 
and  inserting  plank  sheeting  on  the  outside  resting  against  the 
frames ;  or  in  softer  materials,  after  setting  a  frame,  the  sheet¬ 
ing  is  driven  around  it  on  the  outside,  and  driven  ahead  so  as 
to  keep  in  advance  of  the  excavation.  When  the  sheeting 
shows  signs  of  springing  or  bending  another  frame  is  inserted 
and  another  set  of  sheeting  started  between  the  last  frame  and 
the  sheeting  from  above.  With  brick  or  concrete  cylinders 
the  bottom  must  rest  on  a  timber  or  iron  curb,  consisting  of  a 
short  cylinder  of  timber  or  iron  framed  with  a  cutting  edge, 
and  on  top  a  ring  of  timber  or  iron  of  sufficient  width  to  carry 
the  masonry,  and  supported  by  brackets  fastened  to  the  sides 
of  the  curb.  Though  often  used  for  foundations,  such  methods 
are  more  generally  applicable  to  sinking  shafts  in  mining  opera¬ 
tions,  or  in  connection  with  driving  tunnels,  and  constructing 
piers  for  bridges.  Iron  cylinders  were  used  in  founding  the 
City  Hall  of  Kansas  City,  mentioned  in  another  page.  Hollow 
brick,  concrete  and  iron  cylinders  for  piers  of  bridges  will  be 
described  later. 


FOUNDATIONS  FOR  HIGH  BUILDINGS. 


343 


Article  LVI. 

FOUNDATIONS  FOR  HIGH  BUILDINGS. 

76.  In  the  last  few  years  the  construction  of  high  buildings 
in  cities  has  rendered  necessary  a  more  careful  and  thorough 
examination  into  the  bearing  power  of  soils  and  remodelling 
the  underground  columns  and  supports,  so  as  to  secure  safe 
bearing  areas  and  at  the  same  time  so  reduce  their  cross-sec¬ 
tions  that  they  may  occupy  as  little  space  in  the  underground 
compartments  as  practicable ;  and  perhaps  more  thought  has 
been  given  to  this  subject  and  greater  developments  in  this  di¬ 
rection  have  been  made  in  the  city  of  Chicago  than  anywhere 
else.  Until  very  recently  it  has  been  supposed  that  the  clay 
was  underlaid  by  a  thick  layer  of  quicksand  or  some  soft  ma¬ 
terial.  The  practice  has  been  to  guard  carefully  against  cut¬ 
ting  through  this  clay,  and  as  the  heights  of  the  buildings  have 
been  increased  the  bases  of  the  walls  and  column  supports  have 
been  gradually  spread  and  enlarged,  so  as  to  maintain  a  unit 
pressure  not  exceeding  from  3000  to  3500  lbs.  per  square  foot. 
Under  some  structures  piles  have  been  driven,  as  it  was  feared 
that  the  limit  of  safety  had  been  reached  for  sufficient  support 
by  direct  bearing.  This  has  not  been  considered  in  some 
cases  as  entirely  satisfactory  or  even  as  an  improvement 
over  the  former  method,  unless  the  piles  are  made  long  enough 
to  reach  to  the  bed-rock,  which  is  from  50  to  60  ft.  below 
the  surface,  and  are  driven  at  the  bottom  of  excavations  15 
or  more  feet  deep,  so  as  to  insure  that  all  wood-work,  piles, 
caps,  and  flooring,  when  used,  should  be  certainly  below  the 
line  of  constant  moisture.  It  is  claimed  that  careless  driving 
was  the  cause  of  the  inefficiency  of  the  piling.  Following 
upon  this,  Gen.  William  Sooysmith  read  a  paper  arguing  that 
some  method  of  reaching  the  rock  should  be  adopted,  and 
pillars  of  stone  with  polished  beds,  so  as  to  do  away  with  mor¬ 
tar,  should  be  used  to  bring  the  foundation  up  to  or  near  the 
surface  of  the  ground,  claiming  that  such  a  pillar  would  be  four 


344  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 

times  as  strong  as  one  of  ordinary  masonry.  These  methods 
were  argued  against  as  being  entirely  useless  and  unnecessarily 
expensive,  and  the  claim  is  set  forth  that  Chicago  is  underlaid 
by  a  solid  bed  of  compact  clay  from  the  surface  to  or  near  the 
rock,  passing  into  compact  gravel  immediately  above  the  rock, 
and  that  the  borings  thus  far  made  have  been  deceptive  on  ac¬ 
count  of  the  water  which,  though  existing  in  small  quantities 
in  the  clay,  collects  in  the  pits  and  leaves  the  impression  that 
the  underlying  soil  is  either  quicksand  or  mud.  It  seems  to 
the  writer  borings  conducted  as  explained  in  the  second  part 
of  this  volume  would  settle  this  matter  definitely  and  satisfac¬ 
torily,  as  the  material  can  be  brought  up  just  as  it  exists  from 
any  depth,  if  it  is  silt  or  clay.  It  is  admitted  that  proper,  effi¬ 
cient,  and  systematic  borings  have  not  been  made.*  By  these 
parties  it  is  claimed  that  by  the  use  of  the  combined  steel  and 
concrete  beds  a  sufficient  spread  of  base  can  be  obtained  to  bear 
safely  any  height  of  building  likely  to  be  required.  And  so  this 
matter  stands,  ably  argued  on  both  sides,  but,  as  it  seems  to  the 
writer,  without  sufficient  and  reliable  data  being  determined 
to  settle  the  question.  It  will  be  of  interest,  however,  to  see 
what  has  thus  been  accomplished.  The  economizing  in  the 
question  of  cellar  spaces  is  well  illustrated  in  Figs.  15  and  16. 
Fig.  1 5  shows  a  masonry  pier  resting  on  a  concrete  base  ;  Fig.  16 
shows  a  steel  rail  and  concrete  footing  resting  on  an  equal  mass 
of  concrete,  therefore  having  the  same  ultimate  bearing  capacity. 
The  masonry  above  the  concrete  is  7  ft.  high  (see  Fig.  15);  in 
Fig.  16  the  height  is  only  2  ft.  6|  ins.,  the  upper  course  being 
15-in.  eye-beams.  A  similar  construction,  using  rails  for  the 
upper  courses  and  transmitting  the  same  weight,  would  be  only 
1  ft.  8  ins.  high  above  the  concrete.  In  this  case  the  weight  of 
the  masonry  base  is  216,000  lbs.;  the  weight  of  the  steel  base, 
103,000  lbs.  The  weight  on  this  foundation  is  about  800,000  lbs., 
the  weights  of  the  foundations  being  respectively  20  and  13  per 
cent  of  the  total.  The  saving  in  weight  of  the  iron  and  concrete 

*  Since  writing  the  above  Mr.  A.  Gottlieb  has  made  a  large  number  of  bor¬ 
ings;  about  the  average  results  will  be  found  in  the  supplement  to  this  volume. 
A  full  report  was  published  in  the  Engineering  News. 


FOUNDATIONS  FOR  HIGH  BUILDINGS. 


345 


foundations  is  enough  to  allow  an  additional  story.  There  is  also 
a  saving  in  time.  The  cost  of  constructing  the  stone  foundation 
is  a  little  less  than  the  steel,  but  the  increase  of  rental  space 
more  than  compensates  for  this.  The  steel  beams  also  enable 
the  load  to  be  distributed  over  a  part  of  the  area  between  two 


"os 

'0 

1 

1 

U 

5": 

T.  Ji'o" 

1' 

i;  7 

1'.  ,  9" 

=t 

f 

<  16- g"  -  >• 

Fig.  15. — Masonry  Foundations  on  Concrete  Base. 


columns  by  beams  extending  from  the  one  to  the  other,  thereby 
bringing  into  bearing  a  part  of  the  foundation  that  could  not 
be  utilized  for  this  purpose  in  masonry  columns.  The  concrete 
used  is  of  the  best  Portland  cement  and  broken  stone — 1  part 
cement,  2  sand,  and  4  stone.  The  steel  rails  are  75  lbs.  per 
yard.  When  beams  are  used,  the  10,  12,  15,  or  20  in.  beams 
are  best.  The  following  calculation  applies  to  Fig.  16,  the  con¬ 
crete  bed  being  17  ft.  3  in.  X  22  ft.  8  ins.,  somewhat  larger  than 
drawing  :  In  the  eye-beams,  20,000  lbs.  extreme  fibre  strain  is 
allowed,  and  for  the  rails,  16,000  lbs.  The  top  course  is  com¬ 
posed  of  1  5-in.  steel  beams,  50  lbs.  per  foot,  whose  moment  of 


34^  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


resistance  is  117,700  ft. -lbs. ;  the  other  courses  steel  rail,  75  lbs. 
per  yard,  4f  ins.  high  and  an  equal  width  of  base,  having  a 
moment  of  resistance  of  12,100  ft.-lbs.  It  is  required  to  find 
the  projecting  arms  of  the  two  upper  courses.  Those  of  the 
two  lower  courses  are  determined  from  the  lengths  of  the 
upper  ones  and  the  clay  areas  already  determined. 

For  the  two  upper  courses 
y  —  projecting  arm  ; 
l  —  total  load  ; 

a  —  width  of  supported  area ; 

M  —  total  bending  moment  on  one  side  of  the  load. 

Then  total  length  of  beam  —2 y-j-a, 


Total  load  on  y  —  I- 


y 


2y  +  a 

and  as  the  load  in  every  course  is  uniformly  distributed, 


M  — 


ly 


If 


y 

x  —  — 

2y  -j-  a  2  2  (2y  -)-  a) 


=  R. 


(Eq.  I.) 


In  calculating  the  two  lower  courses  y  becomes  the  known 
and  M  the  unknown  quantity.  The  load  on  the  column  is 
1,166,000  lbs.  As  only  nine  beams  can  be  put  under  the 
cast  bed-plate,  M  =  R  =  117,700  lbs.  X  9  =  1,059,300,  then 

1,166,000  ibs.  x  y  r  .  .  , 

-  ■■  ■  ^  -- — —  =  1,059,300  .y  =5  ft.  4  ms. ;  length  of  beam 

—  2y  f  5  =  15  ft.  8  in.  For  the  third  course,  M=R  = 
12,100  X  31  =  375,100  lbs.  This  spaces  the  rails  6  in.  centres. 
The  load  is  1, 166,000  -j-  19,000  (weight  of  top  course  and  con¬ 
crete)  =  i ,  1 85,000  . y  =  2  ft.  6  in.  The  area  covered  by  the 
first  course  must  be  15  ft.  11  in.  X  21  ft.  4  in.,  giving  3  ft.  %  in. 
projection  for  the  first  course  and  2  ft.  10  in.  for  the  second. 
1,200,000  lbs.  X  2|  X  it52 


Then 


—  M  —  225,780.  Requiring 


nineteen  rails  in  course  second,  and 


1,220,000  X  3 in  X  iff 

T  C  1  1 

1  5t^ 


=  M 


=  343,000,  or,  in  the  first  course,  twenty-nine  rails.  Thirty 
were  used.  The  allowable  clay  loads  vary  from  1^  to  2  tons. 


FOUNDATIONS  FOR  HIGH  BUILDINGS. 


34  7 


With  this  load  the  structure  will  settle  from  3  to  5  ins.  After 
carefully  laying  the  rails  and  concrete  the  entire  exposed  sur¬ 
face  is  plastered  over  with  cement  mortar,  so  that  no  part  of 
the  iron  is  exposed.  (See  article  in  Eng.  News,  Aug.  8,  1891, 
by  C.  T.  Purdy,  C.E.)  This  form  of  foundations  is  probably 
the  best  practice  for  high  buildings,  and  therefore  it  is  given 
in  some  detail.  The  great  advantage  of  this  method  is  in  only 
requiring  a  small  thickness  of  concrete.  The  failure  of  the  City 
Hall  was  due  to  a  too  thin  bed  of  concrete  and  being  required 
to  act  as  a  beam,  owing  to  the  unequal  resistance  of  the  clay. 
The  building  settled  unequally,  as  much  as  14  ins. 

The  following  are  examples  of  loads  actually  borne  :  A 
stick  12  ins.  square  on  micaceous  sand  did  not  settle  perceptibly 
under  a  load  of  10  tons.  And  8  tons  per  square  foot  on  screw- 
disks  at  Coney  Island.  East  River  Bridge,  6f  tons  on  sand. 
In  New  York  approach  1600  feet  masonry,  3^  tons  to  4^  tons  ;  no 
cracks.  Clay  under  Capitol  at  Albany,  2  tons  per  square  foot ; 
3  ft.  below  surface.  Bridge  at  London,  on  gravel  over  blue  clay, 
5^  tons  per  square  foot,  failed  after  many  years.  The  Washing¬ 
ton  Monument  when  one  third  built  caused  pressure  of  5  tons 
per  square  foot  on  a  mixture  of  clay  and  sand,  but  settled  after 
a  number  of  years,  if  ins.  out  of  plumb.  The  base  on  resum¬ 
ing  work  was  spread  by  cutting  channels  in  under  the  masonry, 
so  as  to  reduce  the  pressure  to  10,000  lbs.  per  square  foot.  It  is 
estimated  that  this  pressure  is  doubled  on  the  leeward  side  in 
high  winds.  No  evidence  of  further  settling.  Public  works 
in  India  do  not  settle  on  silt  and  alluvium  with  1  ton  per 
square  foot ;  with  2700  lbs.,  settled  f  inch  ;  with  2  tons,  decided 
settlement.  Fort  Livingston,  Mississippi,  built  on  fine  sand, 
20  ft.  thick,  settled  during  building  2  to  3  ft.,  subsequently  1  to 
2  ft.  gradually.  Government  building  at  Chicago  settled  dur¬ 
ing  thirteen  years  6  to  18  ins. 

Tay  Bridge,  Scotland,  on  silty  sand,  load  3  to  3J  tons  per 
square  foot ;  weights  added  increased  the  load  to  5  and  to 
tons  per  square  foot,  remaining  from  6  to  25  days,  settled 
about  if  to  if  ins.  Exhibition  buildings  at  Paris,  gravel  rest¬ 
ing  on  stiff  clay,  6000  lbs.  per  sq.  ft.  when  the  gravel  was  10  ft. 


34§  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


thick,  4550  lbs.  when  5  to  10  ft.  thick  ;  when  less  than  5  ft. 
thick,  piles  were  driven.  In  one  test  a  load  8  tons  per  square 
foot,  caused  a  settlement  of  1 1  ins.  in  12  hours,  but  6  tons  were 
carried  safely.  Hudson  River  Tunnel,  on  Hoboken  side,  safe 
load  on  mud  5580  lbs.  per  square  foot. 

A  length  of  9  ft.  6  ins.  in  a  wall  settled  about  \  inch  with 
a  pressure  of  62  lbs.  per  square  inch  at  time  of  building,  the 
wall  being  built  rapidly.  Loading  walls  too  quickly  has  caused 
bulging.  Building  masonry  with  very  thin  joints  on  the  face 
and  thicker  joints  behind  often  causes  chipping  on  the  face, 
notably  Philadelphia  Public  Building  and  Washington  Monu¬ 
ment  (see  Engineering  News,  Feb.  14,  1891).* 

The  Eiffel  Tower:  total  weight  of  iron,  7300  tons.  The 
total  load  on  foundations  is  565  tons,  increased  to  875  under 
maximum  wind  pressure.  Total  height  of  the  tower,  984  ft. 
There  are  four  independent  foundations  at  the  angles  of  a 
square  330  ft.  on  a  side,  and  each  foundation  is  made  up  of 
four  separate  inclined  piers.  The  main  foundations  are  on  a 
bed  of  gravel  18  ft.  thick,  the  top  of  the  bed  23  ft.  below  the 
surface;  these  rested  directly  on  a  bed  of  concrete  7  ft.  thick. 
P'ortwo  of  the  piers  the  bed  of  sand  and  gravel,  about  40  ft.  be¬ 
low  the  surface,  overlaid  by  soft  deposits,  was  reached  by  the  use 
of  compressed  air,  the  caisson  being  sunk  52  ft.  to  a  good  bear¬ 
ing  soil.  The  bed  stones  under  the  great  piers  have  a  crushing 
strength  of  1600  lbs.  per  square  inch  ;  maximum  load  that  can 
come  upon  them  is  425  lbs.  per  square  inch.  The  total  load  on 
each  of  the  two  foundations  on  the  concrete  bed  is  1970  tons. 
The  concrete  has  the  following  dimensions  :  32  ft.  9  ins.  X  19  ft. 
8  ins.  (  =  644.86  sq.  ft.)  and  6  ft.  6  ins.  thick.  The  load  on  the 
masonry  is  about  3  tons  per  sq.  ft.  (see  Eng.  News,  June  8, 

1  889). 

The  City  Hall  of  Kansas  City  was  constructed  over  the  site 
of  an  old  ravine,  which  was  partly  filled  by  the  material  from 
the  adjacent  clay  bluffs,  and  in  part  by  the  ordinary  rubbish 

*  This  settlement  of  brick  walls  and  buildings  refers  to  settlement  or  shrink¬ 
ing  of  mortar  in  joints  of  masonry  either  causing  settlement  of  whole  structure 
uniformly,  or  unequal  settlement  due  to  difference  in  thickness  of  the  same 
mortar  joint,  throwing  an  excess  of  pressure  on  the  face,  causing  chipping. 


FOUNDATIONS  FOR  HIGH  BUILDINGS. 


349 


dumped  in  from  the  city  carts;  the  fill  was  50  ft.  deep.  Holes 
were  bored  by  means  of  a  large  auger  4  ft.  6  ins.,  worked  by 
steam  ;  an  iron  cylinder,  metal  thickness  j3^  in.,  followed  the 
auger  down ;  when  solid  bottom  was  reached  the  cylinders 
were  filled  with  vitrified  brick  well  bonded  ;  these  bricks  had  a 
crushing  strength  of  about  135  tons  each. 

The  Chicago  Auditorium  Building  has  a  frontage  of  362 
ft.  Total  area  covered  about  63,000  sq.  ft.  The  building 
proper  is  10  stories  high ;  on  one  of  the  fronts  a  tower  rises  240 
ft.,  and  94  ft.  above  the  main  building.  The  foundation  of  the 
tower  covers  an  area  of  69  X  100  ft.  The  weight  on  the  foun¬ 
dation-bed,  15,000  tons,=  about  4350  lbs.  per  sq.  ft.  An  ex¬ 
cavation  was  made  to  the  clay  layer ;  on  this  bed  a  timber 
grillage,  2  ft.  thick,  was  constructed ;  on  this,  solid  concrete  5 
thick  was  placed  ;  and  to  prevent  unequal  settlement  and  dis¬ 
tribute  the  weight  uniformly  three  layers  of  rails,  one  layer 
15-in.  V-beams,  and  one  layer  12-in.  eye-beams  were  imbedded 
in  the  concrete  ;  and  as  an  additional  precaution  against  the 
heavy,  concentrated  weight  of  the  towers  cracking  the  ad¬ 
joining  walls  a  direct  load,  about  equal  to  its  completed  load, 
was  placed  on  the  tower  walls,  and  gradually  removed  as  the 
walls  were  carried  up. 

Many  of  the  high  buildings  have  foundations  of  this  char, 
acter.  Where  timber  is  used  great  care  should  be  taken  to 
place  it  well  below  the  surface  of  constant  moisture. 

In  all  of  the  above  examples,  except  where  specially  noted, 
the  structures  have  stood,  no  serious  settlement  having  oc¬ 
curred  up  to  the  time  of  publication.  Many  other  similar 
examples  could  be  cited,  but  the  above  shows  the  usual  loads, 
methods  of  founding,  and  nature  of  the  structure,  taken  from 
many  localities,  and  constructed  on  many  varieties  of  material. 

The  writer  constructed  the  high  masonry  piers  for  a  bridge 
over  the  Ohio  River  on  this  plan,  bedding,  12  X  12  in.  Michi¬ 
gan  white  pine  in  the  concrete.  Whether  either  timber  or 
iron  is  preserved  from  rot  and  corrosion  when  entirely  cov¬ 
ered  in  concrete  can  scarcely  be  considered  as  a  settled  fact, 
though  generally  accepted  and  believed.  See  Plate  I. 


35°  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


77,  In  the  above  cases  the  pressures  on  the  foundations  are 
in  the  main  given  without  any  complications  or  uncertainties 
resulting  from  frictional  resistances  on  the  sides  of  the  struct¬ 
ures,  which  exist  in  the  case  of  pile  foundations  or  those  con¬ 
structed  by  the  open  crib  or  pneumatic  caisson.  The  published 
accounts  of  these  are  liable  to  be  uncertain  and  misleading, 
both  on  account  of  the  inaccurate  distribution  of  the  total 
resistances  between  the  direct  bearing  resistance  and  the 
frictional  resistance,  which  is  varying  within  wide  limits,  and 
also  from  the  different  manner  of  calculating  the  actual  load 
to  be  supported.  In  sinking  an  open  crib  or  pneumatic 
caisson  some  engineers  deduct  from  the  total  weight  the 
actual  or  assumed  buoyancy  of  the  displaced  water  as  well  as 
that  of  the  displaced  earth,  and  others  do  not.  This  results 
in  a  wide  difference  in  the  resultant  load  to  be  supported  by 
friction  and  direct  bearing.  Some  uniform  method  is  neces¬ 
sary  for  an  intelligent  comparison  or  subsequent  use  in  other 
structures.  In  the  case  of  the  foundations  of  the  Cairo 
Bridge,  already  described,  the  calculation  is  made  as  follows: 


Channel  Piers.  Tons. 

331,000  ft.  B.  M.  timber  at  50  lbs.  per  cu.  ft .  689.6 

Iron .  68.5 

Concrete,  77,345  cu.  ft.  at  145  lbs.  per  cu.  ft .  5*607.5 

Masonry,  102,508  “  “  “  150  “  “  “  “ .  7,688.1 

Superstructure .  1,027.0 

Moving  load .  785.2 


15,865.9 

Deduct  for  displacement  of  78,000  cu.  ft.  sand,  and  22,756 
cu.  ft.  water,  and  frictional  resistance  at  400  lbs.  per 
square  foot . .  9,574-5 

6,291.4 

Assumed  friction  resistance  400  lbs.  per  square  foot. 

Fatigue  weight  6,291.4  tons  =  3.15  tons  per  square  foot.* 

*  In  this  calculation  concrete  at  145  lbs.  per  cu.  ft.  seems  high,  135  lbs.  is 
doubtless  a  good  average.  Masonry  at  150  lbs.  per  cu.  ft.  is  low  for  first-class 
masonry,  average  about  155  to  160  lbs.  Timber  at  4^  lbs.  per  ft.  B.  M.  =  50 
lbs-  per  cu.  ft.  =  2.09  tons  per  1000  ft.  B.  M.  is  a  fair  average.  Taking  sand 
at  130  lbs.,  when  wet,  water  at  62$  lbs.  per  cu.  ft.,  then  (78,000  X  I3°T22>756 
X  62I)  =  5781  tons.  ,\  9574.5  —  5781  ==  3793.5  tons  for  total  frictional  resist¬ 


ance. 


FOUNDATIONS  FOR  HIGH  BUILDINGS. 


351 


Also  for  load  on  concrete  base,  pier  12  (supposed  to  be  a 


pier  on  land) : 

Tons. 

12,072  cu.  ft.  masonry .  905.4 

Superstructure . 234.6 

Moving  load .  379-5 


I.5I9-5 

Or  4.03  tons  per  square  foot. 

The  fatigue  weight  =  total  weight  less  displacement  and  friction. 


From  report  of  Mr.  Geo.  S.  Morrison  on  the  construction 
■of  the  Bismarck  Bridge  (which  contains  much  detailed  and 
valuable  information)  the  following  is  extracted:  Pier  No.  4 
on  land,  excavation  in  sand  to  a  depth  of  about  20  ft.,  and 
piles  driven  in  the  bottom. 


Lbs. 

28,000  ft.  B.  M.  timber  in  curb  at  4  lbs .  112,000 

15,000  “  “  “  “  grillage  at  5  lbs .  75, 000 

264  cu.  yds.  concrete  at  3,510  lbs.  (130  lbs.  per  cu.  ft...  926,640 

1093.3  cu.  yds.  masonry  at  4,330  lbs.  (160  lbs.  per  cu.  ft.  4,733,989 
257  ft.  superstructure  at  5,000  lbs .  1,285,000 


7,132,629 

Deduct  for  immersion  11,390  cu.  ft.  at  62!  lbs .  711,875 


Net  weight . 6,420,754 

i6x  piles,  average  load  per  pile . . .  39,880 


Also  pier  No.  2,  pneumatic  caisson,  sunk  through  water, 
sand,  and  into  a  hard  black  clay.  The  total  weight,  as  calcu¬ 
lated  in  the  same  manner  above,  =  17,269,000,  and  deducting 
for  immersion  4,510,000;  net  weight  =  12,759,000  lbs.;  area  of 
base,  1924  sq.  ft.;  average  pressure  per  square  foot,  6631  lbs., 
or  46  lbs.  per  square  inch. 

In  the  first  case  a  steam  pile-hammer  was  used  in  driving 
the  piles :  depth  driven  in  sand  varied  from  23  to  34  feet. 
The  penetration  in  the  last  10  blows  varied  from  o  to  0.2  of  a 
foot,  or  an  average  for  the  greater  penetration  of  0.02  ft. 

Similarly  in  the  Plattsmouth  Bridge  one  of  the  piers  on  a 
pile  foundation  has  the  following  record:  Total  weight, 
.2,114,750  (apparently  no  deduction  for  immersion);  number 


352  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


of  piles,  78;  average  weight  on  pile,  27,112  lbs.  The  piling 
record  is  especially  interesting.  Weight  of  hammers,  3100 
and  3900  lbs.;  average  fall  at  last  blow,  28  ft.;  average  pene¬ 
tration  at  last  blow  about  if  ins.,  some  few  as  much  as  2  to  3 
ins.  ;  average  depth  in  the  sand,  27  ft.  Out  of  the  78  piles,  8 
broke  off  under  the  blows;  15  piles  broke,  mashed,  and  split  or 
otherwise  injured  ;  that  is,  about  10  per  cent  were  broken  off 
and  about  20  per  cent  were  visibly  injured.  The  number  of 
blows  ranged  from  ico  to  142  per  pile.  It  can  hardly  be 
doubted  that  there  were  many  piles  more  or  less  seriously 
crippled  below  the  surface  and  out  of  sight. 

The  other  piers  being  founded  on  rock,  the  pressure  per 
square  foot  is  unimportant.  They  ranged,  however,  from  4090 
lbs.  to  6393  lbs.  per  square  foot.  No  allowance  for  nor  notice  of 
frictional  resistance  seems  to  have  been  allowed  for  in  these 
reports.  Although  Mr.  Morrison  deducts  for  the  immersion  of 
the  structure,  he  says  that  this  is  only  done  to  get  the  rela¬ 
tive  pressure,  that  is,  the  increased  pressure  on  the  foundation 
over  that  on  the  surrounding  surface,  “which  is  the  real 
measure  of  the  labor  of  the  foundation.”  The  actual  pressure 
is  the  whole  weight  of  the  structure  “with  the  addition  of  the 
atmospheric  pressure.”  The  relative  pressure  might  be  useful 
in  estimating  the  bearing  resistance  at  one  depth  in  a  certain 
material,  knowing  the  bearing  resistance  at  any  other  depth, 
provided  the  law  of  variation,  increase,  or  decrease  was  pro¬ 
portional  to  the  displacement ;  but  this  could  only  be  true  in 
a  perfect  fluid.  If  this  is  true  no  allowance  for  side  friction 
should  ever  be  made.  Some  engineers  do  not  consider  friction 
at  all,  owing  to  the  fact  that  it  may  be  in  part  or  entirely 
destroyed  by  scour.  That  it  does,  however,  form  a  very  large 
proportion  of  the  actual  bearing  resistance  of  many  structures, 
and  often  is  the  sole  reliance  cannot  be  denied.  Other  examples 
of  pressure  on  foundation-beds  have  been  given  in  the  preced- 
ing  pages. 

78.  The  determination  of  the  frictional  resistance  on  the 
surfaces  of  piles,  cribs  and  caissons  has  not  been  considered 
as  carefully  as  the  importance  of  the  subject  demands.  Favor- 


FOUNDATIONS  FOR  HIGH  BUILDINGS. 


353 


able  conditions  do  not  always  present  themselves,  and  exact 
conditions  are  not  always  known,  friction  during  continuous 
motion  being  different  from  that  developed  in  starting  from  a 
condition  of  rest,  and  again  varying  with  the  period  of  rest ; 
and  often  high  average  surface  resistance  may  result  from  great 
local  resistance  at  only  a  few  points.  The  following  are  a  few 
examples  of  the  estimated  frictional  resistances  by  different 
engineers,  from  data  and  other  considerations  satisfactory  to 
them.  But  to  be  convinced  of  the  value  and  importance  of 
friction,  endeavor  to  pull  piles  and  find  the  power  necessary, 
after  due  allowance  for  weight  of  pile,  the  effect  of  suction, 
and  want  of  rigidity  in  the  fulcrum,  and  this  too  notwithstand¬ 
ing  that  smaller  cross-sections  are  being  exposed,  and  only 
after  a  considerable  amount  of  lift  will  the  pile  be  raised 
readily  and  rapidly.  On  measuring  friction  of  motion  by  sink¬ 
ing  piles  with  weights,  the  loss  of  frictional  resistance  is  not 
so  apparent,  as  larger  surfaces  are  being  pressed  where  the 
smaller  were  ;  and  this  largely  explains  the  fact  that  pile  founda¬ 
tions  generally  settle  only  a  short  distance  before  a  new  condition 
of  equilibrium  is  brought  about.  And  as  is  often  stated  that  it 
is  only  the  initial  resistance  to  settling  that  is  worth  consider¬ 
ing,  it  is  evident  that  the  above-mentioned  fact  has  been  over¬ 
looked.  In  sinking  a  pneumatic  caisson  or  crib,  as  smaller  sur¬ 
faces  are  being  continually  presented  in  the  place  of  larger  ones, 
the  initial  resistance  is  soon  dissipated  entirely,  and  the  ten¬ 
dency  is  to  continue  going  when  once  started.  This  is  entirely 
in  keeping  with  the  experience  in  the  Hawkesbury  caissons, 
where  those  without  any  bottom  spread  were  the  more  readily 
handled,  and  gave  less  trouble.  In  measuring  the  resistance 
to  sinking  caissons,  it  is  the  exception,  rather  than  the  rule, 
that  the  compressed  air  is  entirely  removed  from  the  caisson. 
This  would  reduce  the  resistance  by  the  air-pressure  per  square 
inch  or  square  foot.  The  escape  of  the  compressed  air  under 
the  caisson  may  materially  reduce  the  outside  friction.  In 
many  cases,  however,  the  air  finds  a  passage  of  escape  at 
considerable  distances  from  the  caisson,  as  evidenced  by  the 
bubbling  of  the  water  at  the  surface.  It  is  clear,  then,  that. 


354  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


estimates  made  on  the  frictional  resistance  cannot,  under  the 
ordinary  conditions  and  manner  in  which  they  are  calculated, 
be  regarded  as  by  any  means  accurate.  But  more  careful  and 
extended  observations  should  be  made  when  opportunities 
occur.  The  circumstances  under  which  the  following  estimates 
were  made  are  not  known  to  the  writer,  except  those  made  by 
himself.  It  is,  however,  all  that  he  has,  and  is  given  simply  as 
stated  in  Engineering  News  and  other  works.  Mr.  Collinwood 
in  the  News  of  Feb.  21,  1891,  states  that  in  sinking  brick  wells 
5  to  18  ft.  diameter  to  a  depth  of  50  ft.  or  more,  a  load  of  300 
tons  was  required  ;  in  the  alluvium  in  India  the  “  skin  friction” 
was  from  500  to  1500  lbs.  per  square  foot.  At  the  Dufferin 
Bridge  it  was  1000  lbs.  for  wells  12^  ft.  in  diameter.  On  the 
caissons  of  iron  of  the  St.  Charles  Bridge,  sunk  through  20  ft. 
of  bowlders,  the  friction  was  4 66  lbs.  per  square  foot.  The 
caissons  of  wood,  East  River  Bridge,  gave  about  900  lbs.  in 
bowlders,  clay,  and  sand,  and  in  clear  sand  from  400  to  600  lbs. 
per  square  foot.  And  in  case  of  iron  caisson  for  a  lighthouse, 
200  lbs.  per  square  foot. 

In  the  case  of  caisson  No.  2,  Susquehanna  River,  sunk  under 
the  writer’s  supervision  :  Indicated  air-pressure  of  33  lbs.  This 
was  reduced  to  26  lbs.  in  order  to  sink  the  caisson  under  a  total 
weight  of  9,356,760  lbs. ;  deducting  the  buoyant  effect  of  the 
air  =  6,143,904  lbs.,  the  net  weight  to  overcome  resistance  = 
3,212,856  lbs.,  giving  331^-  lbs.  per  square  foot  of  exposed  sur¬ 
face  in  sand.  A  subsequent  test  gave  380  lbs.  per  square  foot. 
At  this  depth  the  caisson  had  entered  into  a  layer  of  bowlders 
which  continued  as  far  as  the  caisson  was  sunk,  68'  4"  below 
low  water,  about  55  ft.  through  a  rather  clean  sand  and  under¬ 
lying  bowlders.  The  material  was  entirely  removed  from  under 
the  caisson,  and  it  was  held  by  friction  alone.  The  only  source 
of  error  in  the  last  case  (380  lbs.)  was  inaccuracy  in  the  pressure- 
gauge.  The  indicated  pressure  was  reduced  to  that  due  to  the 
depth ;  this  was  done  as  in  ordinary  sand  and  gravel  it  is  diffi¬ 
cult  to  maintain  a  pressure  greater  than  that  due  to  the  depth, 
on  account  of  leakage.  Had  a  similar  reduction  (5^  lbs.,  gauge 
36  lbs. ;  depth  below  water  about  60  ft. ;  estimated  actual  press- 


FOUNDATIONS  FOR  HIGH  BUILDINGS. 


355 


ure  304  lbs.)  been  made  in  the  first  case,  the  two  records  (33 14 
and  380  lbs.)  would  have  been  nearer  together,  as  they  should 
have  been.  The  weights  were  the  same,  and  the  depths  being 
44^  and  4 7  ft.,  respectively.  In  sinking  caisson  No.  3  the  two 
records  show  at  a  depth  of  about  40  ft.  below  the  bed  of  the 
river  in  good  grained  sand,  weight  8,465,871  lbs. ;  buoyant 
effect  of  air,  6,032,989  lbs.,  reduced  weight  to  2,432,882  lbs. ; 
area  of  surface  below  the  bed  of  the  river,  8533  square  feet; 
hence  friction  resistance  per  square  foot  285  lbs.  When  464 
ft.  down  record  shows  379  lbs.,  having  entered  the  bowlders. 
The  caisson  was  sunk  about  4  ft.,  and  building  commenced  on 
the  bowlders.  Depth  below  bed  of  river  50'  8f".  Total  below 
water  surface  70'  8£". 

At  caisson  No.  4,  depth  of  water,  28  ft.  at  low-water; 
depth  of  solid  material  to  highest  point  of  rock,  31'  io4// to 
owest,  37'  3^// ;  which  was  a  compact  silt,  air-tight  and  water¬ 
tight,  but  easily  forming  mud  or  slush  when  mixed  with  water. 
When  the  caisson  first  rested  on  the  bottom  (as  it  had  a  false 
bottom  for  launching  ;  none  of  the  other  caissons  had  one),  it 
rested  easily,  but  sunk  3.3  ft.  while  cutting  out  false  bottom. 
The  ordinary  weighting  with  timber  and  concrete  was  sufficient 
to  sink  the  caisson  into  the  bed  about  14  ft.  by  only  reducing 
the  pressure  about  3  lbs.  ;  and  when  the  cutting  edge  was 
57.5  ft.  below  water,  or  29.5  ft.  in  the  silt,  the  caisson  settled 
under  a  reduction  of  only  1  lb.  in  the  air-pressure,  showing  a 
nice  adjustment  between  weight  and  resistances.  At  this  depth, 
total  weight  10,958,448  lbs.;  upward  pressure  of  air,  9,014,907 
lbs.;  resistance  to  sinking,  1,943,541,  giving  308  lbs.  per  square 
foot.  In  settling  1.3  ft.  farther,  one  edge  of  the  caisson  rested 
on  rock  ;  and  as  the  caisson  was  out  of  level  about  1 5  inches,  the 
rock  was  blasted  off  along  this  end  ;  then  reducing  the  pressure 
to  level  the  caisson,  the  frictional  resistance  was  apparently 
4894  lbs.;  but  the  caisson  was  blocked  against  the  soft  mate¬ 
rial  on  the  lower  side  of  the  caisson  to  prevent  settling  there ; 
this  record  is  then  too  high.  The  caisson  was  stopped  at 
65'  3i".  below  low-water;  the  excavation  carried  on,  without 


356  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


difficulty,  below  the  cutting  edge  to  rock  on  all  sides.  Great¬ 
est  depth  to  rock,  69'  10 . 

In  caisson  No.  8,  depth  of  water  29  ft.;  of  soft  silt  at  high¬ 
est  point  of  rock,  47  ft.;  at  lowest,  59.34  ft.  Caisson  was  only 
sunk  3  or  4  ft.  into  rock  at  highest  point.  Excavation  carried 
on  below  cutting  edges,  exposing  rock  over  whole  area.  Least 
depth  below  low-water,  76  ft.;  greatest,  88.34  ft.  Owing  to 
the  large  size  of  this  caisson,  the  softness  of  the  silt,  and  ap¬ 
prehension  of  trouble,  the  material  was  never  removed  entirely 
from  under  the  caisson  ;  in  fact,  it  was  sunk  resting  on  block¬ 
ing  to  a  great  extent,  so  it  was  impossible  to  estimate  the  fric¬ 
tional  resistance  with  any  degree  of  accuracy.  The  caisson 
sunk  almost  continuously,  although  concreting  was  stopped 
when  within  29  ft.  of  the  rock,  the  only  extra  weight  being 
the  necessary  timder  to  keep  the  top  above  water  as  the  cais¬ 
son  settled.  The  frictional  resistance  could  not  have  ex¬ 
ceeded  200  lbs.  to  the  square  foot,  owing  to  the  slimy  nature 
of  the  material  passed  through.  The  depth  below  high  tide 
would  be  about  92  ft.  The  other  depths  for  caissons  2,  3,  and 
4  should  also  be  increased  by  the  same  amount,  as  this  latter 
was  the  depth  for  which  air-pressure  had  to  be  provided  every 
day.  In  high  winds  the  tides  would  be  either  very  high  or 
very  low.  The  following  observations  were  made  in  sinking 
the  caissons  for  the  Cairo  Bridge  : 

Penetration  in  sand,  86.42  ft.;  below  water  surface,  90.27  ft.;  weight  of  caisson, 

887  tons  :  crib,  3163  tons  ;  masonry,  2800  tons  ;  weight  of  sand  and 

water,  1806  tons  ;  total,  8656  tons. 

Indicated  air-pressure  before  sinking .  42.75  lbs. 

Calculated  “  “  “  “  .  39.117  “ 

Indicated  “  “  “  “  when  lowered....  36.00  “ 

Air-pressure . 42.75,  39.117,  36.00  “ 

“  “  reaction  due  to  . 4,802,  4,394*  4,044  tons 

Net  weight . 3.854,  4,262,  4,612  “ 

Exposed  surface  of  caisson . 12,910,  12,910,  12,910  sq.  ft. 

Frictional  resistance . 597,  660  715  lbs.  per  sq.  ft. 

However,  in  finding  the  “  fatigue  ”  weight  (see  par.  77), 
or  pressure  on  the  foundation  bed  =  total  weight  —  deductions 


FOUNDATIONS  FOR  HIGH  BUILDINGS . 


357 


for  displacement  of  sand  and  water  and  frictional  resistance 
on  exposed  surface  of  caisson,  they  only  allowed  400  lbs. 
friction  per  square  foot.  Assuming  wet  sand  to  weigh  140 
lbs.  per  cubic  foot,  and  water  62 \  lbs.,  we  find  the  weight 
of  the  displaced  material  =  78,000  X  140  lbs.  -f-  22,75 6  X  62 £ 
—  6171  tons,  and  side  friction  =  9574.5  — 6171  tons  =  3403. 5 
tons,  which  would  give,  at  400  lbs.,  17,017.5  square  feet  of  sur¬ 
face  in  that  case.  The  data  is  not  given,  but  this  shows  the 
method  of  calculation  adopted. 

79.  There  seems  to  be  very  few  records  in  regard  to  the 
frictional  resistance  on  the  surface  of  piles.  In  what  is  in  fact 
a  liquid  mud  the  resistance  to  settling  has  been  fully  shown  to 
be  not  less  than  130  lbs.  per  square  foot,  and  in  compact  silts 
and  clays  200  to  250  lbs.  would  not  be  excessive,  though  not 
based  upon  actual  experiment.  As  in  sinking  caissons,  which 
would  certainly  give  minimum  values  for  the  reason  stated, 
300  to  400  lbs.  per  square  foot  is  a  fair  resistance,  and  as  this 
increases  as  piles  settle,  we  can  safely  allow  from  300  to  500 
lbs.  for  piles  in  sand  and  gravel. 

In  a  letter  from  the  city  engineer  of  New  Orleans  he  states 
that  the  soil  is  alluvial — a  sandy  clay  saturated  with  water  at 
a  depth  of  three  or  four  feet.  Allowing  1000  to  1500  lbs.  per 
square  foot,  if  the  spread  is  not  more  than  ten  bricks,  the  brick 
wall  is  simply  started  on  the  bottom  of  the  trench.  If  piles 
are  required,  they  are  driven  4  ft.  centres  and  capped  with 
a  four-inch  floor,  upon  which  the  brick-work  is  started. 
Piles  from  25  to  40  ft.  long  will  carry  from  15  to  25  tons, 
with  a  factor-of-safety  of  6  to  8.  Taking,  then,  a  pile  of 
25  ft.  into  the  ground,  and  assuming  average  diameter  12  ins., 
having  then  3  square  feet  to  the  linear  foot,  or  75  square 
feet  of  surface,  the  direct  bearing  being  taken  at  1500 
lbs.,  the  safe  load  on  the  pile  will  be  15  X  2000=30,000 
lbs.,  or  to  be  carried  by  friction,  30,000 —  1500=28,500  lbs., 
and  frictional  resistance  per  square  foot  will  be  380  lbs.,  and  for 
the  40-ft.  piles  405  lbs.  per  square  foot.  This  is  in  excess  of 
the  suggested  allowance  by  405  —250=155  lbs.,  for  such 
material. 


358  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


The  enormous  grain  elevators  in  Chicago  rest  upon  pile 
foundations.  Mr.  Adler  states  that  the  unequal  and  constantly 
shifting  loads  are  a  severer  test  upon  the  foundations  than  a 
static  load  of  a  20-story  building.  Taking  the  load  on  the  steel 
and  concrete  piers  already  illustrated,  the  concrete  bed  is  17  ft. 
3  in.  X  22  ft.  8  ins. ;  with  piles  2.8  ft.  centres  we  could  get  six 
rows  of  nine  piles  each  =  54  piles;  with  3  sq.  ft.  per  linear  foot 
a  50-ft.  pile  would  expose  150  sq.  ft.  of  surface  at  a  frictional 
resistance  of  only  100  lbs.;  each  pile  would  carry  15,000  lbs., 
and  the  54  piles  would  carry  810,000  lbs.  On  the  concrete  and 
steel  foundation  the  load  is  800,000  lbs.;  at  ft.  centres  about 
70  piles  could  be  driven  in  the  same  space.  With  the  low 
limit  of  77  lbs.  per  square  foot  a  50-ft.  pile  would  carry  1 1,550 
lbs.,  and  the  70  piles  808,500  lbs.  With  the  knowledge  that  we 
have  there  can  be  but  little  doubt,  if  any,  that  a  pile  founda¬ 
tion  will  carry  any  loads  yet  put  upon  the  soil  underlying  the 
city  of  Chicago,  if  properly  arranged  and  driven. 

80.  The  reader  will  have  learned,  if  he  has  even  casually 
glanced  over  this  volume,  that  engineers  and  architects  are 
far  from  agreement  as  to  the  mode  of  determining  the  bear¬ 
ing  power  of  any  of  the  materials  upon  which  we  have  to 
build  ;  we  are  far  from  agreement  as  to  the  safe  loads  that  can 
be  put  upon  them  or  the  proper  manner  of  distributing  the 
loads.  In  such  cases  arguments  are  useless  ;  high-sounding  or 
ingenious  formulae  are  of  but  little  value.  Theories  will  not 
solve  the  problem. 

What  we  need  is  systematic,  honest,  extensive  experiments 
and  tests,  and  with  these,  honest,  impassionate  interchange 
of  ideas  and  deductions,  without  petty  jealousies  or  fault-find¬ 
ing.  With  rigid  but  kindly  criticism  of  designs  and  of  methods 
of  construction,  we  might  hope  to  advance  our  knowledge, 
improve  our  practice,  and  give  the  public  safe,  substantial,  and 
satisfactory  results,  at  the  least  cost  and  in  the  least  time. 

In  writing  this  volume  the  writer  has  endeavored  to  avoid 
putting  forward  pet  plans  or  theories.  If  prominence  has  been 
given  to  designs,  it  is  only  because  he  believed  them  as  good  as 


FOUNDATIONS  FOR  HIGH  BUILDINGS. 


359 


those  of  others,  and  was  more  familiar  with  their  details ;  and 
with  simple  alterations  in  some  of  the  details  they  are  typical 
of  all  such  structures.  He  has  commented  on  the  designs  of 
others,  expects  such  criticisms  of  his  own,  as  only  in  this  way 
can  we  hope  to  arrive  at  the  truth. 

Article  LVII. 

CONCLUSIONS. 

81.  The  increasing  demand  for  high  buildings,  owing  to  the 
contracted  areas  upon  which  buildings  must  be  erected  and 
the  enormous  cost  of  the  same,  has  naturally  led  to  much  dis¬ 
cussion,  many  eminent  engineers  and  architects  claiming  that 
the  limiting  height  and  consequent  weight  per  square  foot  of 
bearing  surface  has  been  reached.  An  additional  spread  of 
base  beyond  that  now  attained  is  impossible  on  account  of  the 
contracted  areas  and  also  on  account  of  the  rights  of  abut¬ 
ting  property-owners.  And  that  owing  to  the  already  great 
unit  pressures  on  the  usual  foundation-beds  of  sand,  gravel,  or 
clay,  which,  although  they  now  apparently  safely  carry  their 
loads  without  any  but  a  very  small  and  allowable  settlement, 
yet  such  structures  may  be  regarded  as  in  a  precarious  condi¬ 
tion  if  subsequent  operations  of  abutting  property-owners, 
such  as  tearing  down  existing  buildings  and  excavating  to 
greater  depths,  either  for  increased  cellar  room  or  to  secure 
better  and  stronger  foundations,  should  remove  lateral  support 
from  the  foundation-bed  of  an  adjacent  building.  The  result 
would  be  a  flow  or  bulging  of  the  material,  and  thereby  causing 
serious  cracks  or  other  permanent  injury  to  the  building. 
Especially  is  this  a  living  danger  if  the  material  is  a  water¬ 
bearing  sand  or  silt. 

82.  The  substitution  of  piles  as  a  means  of  spreading  the  base 
or  acquiring  increased  bearing  resistance  is  advocated  by  many 
equally  eminent  engineers  and  architects,  backed  by  many 
well-established  precedents.  On  the  contrary,  however,  ex¬ 
amples  of  structures  on  pile  foundations  are  not  wanting  to 
show  that,  for  some  not  explained  or  inexplicable  reason,  after 


360  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


carrying  a  load  for  years  with  perfect  safety,  such  foundations 
have  ultimately  failed,  causing  either  a  partial  or  complete 
wreck  of  the  structure. 

Whether  or  not  pile  foundations  are  any  better,  so  far  as 
affording  direct  support  is  concerned,  may  be  a  matter  of  rea¬ 
sonable  dispute  and  difference  of  opinion,  as  piles  may  be 
badly  injured  in  driving,  their  upper  portions  may  not  be 
driven  below  surfaces  of  constant  moisture,  or,  if  so  driven, 
the  superincumbent  weight  may  lower  the  level  of  this  water- 
surface  ;  or  subsequent  systems  of  drainage,  or  even  natural 
subsidence  of  the  surface,  may  occur,  exposing  the  piles  to 
conditions  of  alternate  wetness  and  dryness,  resulting  in  the 
piles  rotting  and  consequent  failure  of  the  structure.  A  case 
of  this  kind  occurred  under  the  tower  of  a  market-house  in  the 
city  of  Richmond.  The  building  was  badly  cracked.  Col.  W.  E. 
Cutshaw,  City  Engineer,  on  excavating  below  the  structure 
found  that  it  had  been  originally  built  on  piles,  which  had 
rotted  to  a  considerable  extent  and  depth  below  the  surface. 
After  forcing  the  walls  back,  closing  thereby  the  cracks  to  a 
great  extent,  and  supporting  them  in  this  position  with  jacks 
and  props,  he  excavated  under  the  walls  and  filled  the  trench 
with  a  carefully  made  cement  concrete,  rammed  in  layers,  which 
were  allowed  to  partially  set  as  the  work  progressed,  so  that 
when  the  concrete  was  rammed  under  and  against  the  old 
walls  the  entire  mass  had  a  good  set.  Subsequently  the  props 
were  removed,  throwing  the  entire  weight  on  the  concrete. 
No  further  trouble  has  occurred. 

83.  Although  this  and  similar  cases  show  the  uncertainty, 
under  some  conditions,  of  pile  foundations,  they  are  used  to  a 
great  extent  with  entire  satisfaction,  and  will  certainly  to  a 
very  great  extent  do  away  with  the  danger  of  the  material 
flowing  or  bulging  from  under  a  structure  by  removing  the 
weights  from  or  excavating  in  adjoining  lots. 

We  have,  then,  open  to  the  builder  for  selection: 

1st.  Simply  building  the  foundation  walls  or  pillars  on  the 
natural  bed,  spreading  the  base  with  projecting  courses  of 
masonry. 


FOUNDATIONS  FOR  HIGH  BUILDINGS.  36 1 

2d.  Obtaining  the  necessary  spread  with  a  timber  platform 
or  grillage. 

3d.  Driving  piles,  either  to  some  hard  or  firm  material  or 
to  rock. 

These  two  methods  may  cause  settlement  by  rotting  of  the 
timber. 

4th.  Building  the  walls  upon  beds  of  concrete  of  sufficient 
area,  either  alone  or  strengthened  by  iron  or  timber  beams 
built  in  the  concrete. 

5th.  Sinking  cylinders  of  iron  or  caissons  of  timber  or  iron 
of  such  dimensions  as  to  support  either  a  single  column  or  a 
series  of  columns  or  walls,  these  caissons  being  sunk  either  to 
rock  or  to  such  a  depth  and  material  as  will  preclude  the  pos¬ 
sibility  of  failure  occurring  from  any  of  the  above-mentioned 
causes. 

84.  Two  notable  examples  of  this  last  method  are  found  in 
the  City  Hall  of  Kansas  City,  in  which  case  cylinders  were  sunk 
to  rock,  as  explained,  and  the  new  pump-house  of  Louisville 
Water-works,  which  is  erected  on  a  large  timber  caisson  sunk 
by  the  pneumatic  process,  a  single  caisson  of  sufficient  dimen¬ 
sions  for  the  entire  building  to  rest  on  being  used. 

35.  A  somewhat  new  departure  in  this  direction  will  be 
found  in  case  of  a  large  building,  the  Manhattan  Life  Building, 
shortly  to  be  erected  in  New  York.  Sooysmith  &  Co.  are  the 
contractors,  and  they  have  kindly  sent  me  in  advance  of  the 
commencement  of  the  work,  or  even  the  construction  of  the 
caissons,  the  following  facts  and  data.  My  excuse,  if  any  is 
needed,  for  inserting  any  account  of  a  foundation  not  even 
commenced  in  a  work  on  foundations  is  the  perfect  assurance 
that  the  work  will  be  carried  to  a  satisfactory  completion, 
whatever  may  be  the  difficulties  or  costs  involved,  by  the 
contractors. 

Depth  to  bed-rock  is  50  ft.  below  the  level  of  Broadway 
and  25  ft.  below  the  cellar  floor,  which  is  even  with  the  natural 
water-level.  The  caissons  will  be  sunk  by  the  pneumatic 
process  through  sand  and  quicksand.  There  will  be  fifteen 
caissons  built  of  boiler-plate  steel.  Four  (4)  of  the  caissons 


3 62  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 

will  have  circular  cross-sections,  varying  from  io  to  15  ft.  in 
diameter.  The  remaining  eleven  (11)  caissons  will  be  of  rec¬ 
tangular  cross-section,  ranging  in  size  from  13  to  26  ft.  square, 
some  of  them  being  10  ft.  X  26  ft.  in  dimensions.  The  roof 
will  be  strengthened  by  15-in.  eye-beams.  The  piers  will  con¬ 
sist  of  hard-burnt  bricks  laid  in  Portland  cement,  capped  with 
severed  courses  of  granite,  stepped  off  to  a  proper  size  to  re¬ 
ceive  the  bed-plates  for  the  iron  columns,  some  of  which  will 
support  as  much  as  1500  tons. 

The  entire  number  of  caissons  will  be  put  in  place  at  once 
and  the  brick-work  commenced  on  them.  Several  will  be 
sunk  at  the  same  time.  It  is  the  intention  of  the  contractors 
to  sink  these  caissons  with  a  weight  sufficient  to  overcome 
both  frictional  resistance  and  the  buoyant  effect  of  the  com- 
piessed  air,  and  not  to  sink,  as  is  ordinarily  done,  by  reducing 
the  air-pressure.  The  object  of  this  is  to  prevent  the  large 
inflow  of  sand  and  gravel  which  usually  takes  place  when  the 
air-pressure  is  reduced. 

It  is  highly  probable  that  this  can  be  accomplished,  as  the 
depth  to  be  sunk  is  not  very  great,  and  both  the  total  frictional 
resistance  and  upward  pressure  of  air  will  be  relatively  small. 

At  veiy  gieat  depths  in  sand  and  gravel  it  becomes  often 
difficult  to  sink  the  caisson,  even  with  greatly  reduced  air- 
pi  essure,  and  in  many  sands  and  gravels  more  or  less  inflow 
of  the  material  takes  place  even  against  the  air-pressure,  espe¬ 
cially  if  the  excavation  is  carried  at  all  below  the  cutting  edge. 
At  least  this  is  the  writer’s  experience.  The  ability  and 
experience  of  the  contractors  will,  however,  enable  them  to 
contend  successfully  with  any  difficulties  likely  to  arise. 

The  entire  question,  heretofore,  of  applying  this  method 
to  ordinary  buildings  has  been  one  of  actual  and  relative 
rapidity  and  cost  in  the  construction.  If  these  difficulties  can 
be  removed,  there  can  be  no  question  of  the  advisability  of 
adopting  this  method  for  all  important  buildings,  even  if  the 
necessity,  from  a  practical  point  of  view,  does  not  require  it — 
certainly  when  rock  is  at  no  very  great  depth  below  the  sur¬ 
face,  as  the  feeling  of  perfect  safety  as  well  as  the  demand 


FOUNDATIONS  FOR  HIGH  BUILDINGS.  363. 

for  it  will  justify  the  increased  cost,  provided  it  is  not  too 
large  a  percentage  of  the  entire  cost  of  the  structure.  And  as- 
buildings  become  more  costly  the  smaller  will  be  the  percentage 
of  cost  for  the  foundation,  and  the  greater  is  the  reason,  purely 
from  a  selfish  or  economic  point  of  view,  to  make  the  founda¬ 
tions  absolutely  safe  and  secure,  to  say  nothing  of  the  greater 
danger  to  the  comfort  as  well  as  to  the  life  of  the  occupants, 
which  imposes  a  far  larger  responsibility  on  the  builders. 

86.  In  this  connection  the  relative  advantages,  both  .as 
regards  rapidity  and  cost  of  construction,  of  sinking  wells  for 
foundations,  which  is  done  successfully  in  India  to  a  great  ex¬ 
tent,  and  to  which  allusion  has  already  been  made,  may  be 
discussed.  An  interesting  construction  of  this  kind  has  been 
only  recently  completed,  an  account  of  which  will  be  found  in 
the  Engineering  News  of  January  12,  1893.  These  foundations 
were  for  the  piers  of  a  bridge  on  the  Madras  Railway,  India. 
The  method,  however,  is  equally  applicable  to  the  foundations, 
of  high  buildings  where  it  is  desired  to  reach  bed-rock.  The 
spans  for  this  bridge  were  about  140  ft.  long.  The  masonry 
for  the  abutments  and  piers  above  a  certain  level  was  lime¬ 
stone,  resting  on  foundations  obtained  by  well-sinking. 

The  plan  of  the  abutment  was  of  the  usual  form,  with  face 
wall  and  splaying  wings.  Seven  masonry  wells  resting  on 
curbs  were  sunk  to  rock  57  ft.  below  the  bed  of  the  river — 
three  wells  under  the  face  wall  and  two  under  each  wing. 
The  external  diameter  of  the  well  was  12  ft.  where  it  rested 
on  the  curb,  and  for  a  height  of  20  ft.  above,  where  it  was 
reduced  to  11  ft.  diameter.  The  walls  of  the  well  were  built 
of  limestone  masonry  laid  in  mortar.  In  one  of  the  wells, 
sunk  40  ft.  through  clay,  one  side  rested  on  masonry  of  an 
adjacent  well,  and  the  other  side  on  a  curb  of  another  adjacent 
well,  but  the  excavation  was  carried  down  between  the  wells 
to  the  bed-rock,  and  the  space  refilled  with  concrete.  The 
entire  enclosed  space  in  all  of  the  wells  was  filled  with  Port¬ 
land  cement  concrete.  The  thickness  of  the  walls  of  the  wells 
is  not  given,  but  assuming  from  2\  to  3  ft.,  the  diameter  of  the 
enclosed  hollow  space  would  be  from  7  to  8  ft.,  and  about  57 


364  A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


ft.  long.  Arches  were  then  sprung  from  cylinder  to  cylinder, 
and  the  solid  masonry  built  on  the  arches.  The  piers  were 
constructed  by  sinking  two  cast-iron  cylinders,  12  ft.  in  diameter, 
for  each  pier  placed  18  ft.  centre  to  centre.  These  cylinders 
were  from  40  to  47  ft.  high,  reaching  from  bed-rock  55  to  62 
ft.  below  the  bed  of  the  river,  to  a  point  15  ft.  below  the  bed. 
At  this  point  the  diameters  were  reduced  by  a  conical  taper 
7  ft.  6  ins.  high  to  9  ft.,  and  carried  to  full  height  at  this 
diameter.  These  were  finished  on  top  with  a  sliding  cap  and 
connected  by  wrought-iron  massive  bracing-boxes,  bolted  and 
'''filled  with  concrete.  To  facilitate  the  sinking  of  the  cylinders 
at  a  point  12  ft.  above  the  cutting  edge  brackets  2  ft.  9  ins. 
wide  were  fastened  to  the  cylinders,  on  which  a  masonry  wall  or 
lining  was  built  up  to  a  point  at  the  middle  of  the  length  of  the 
conical  taper.  This  really  converted  the  cylinder  proper  into  a 
masonry  well  with  iron  casing,  reducing  the  external  frictional 
resistance.  The  hollow  spaces  in  all  cylinders  were  then  filled 
with  concrete.  Above  the  top  of  these  wells  or  cylinders  solid 
masonry  was  built  to  the  top  of  the  pier,  below  the  bed  of  the 
river  in  cement  mortar,  and  above  in  “  surki  ”  mortar.  The  total 
depth  sunk,  of  cylinders  and  wells,  was  2364  ft.  The  cost  for 
the  cylinders  was  $14,170,  and  for  the  wells  $11,630,  including 
charges  for  bedding  on  the  rock  performed  by  divers.  Total 
cost,  $306,402,  or  $146  per  linear  foot  of  bridge,  which  was 
2100  ft.  long.  The  iron  in  the  cylinders  cost  $85,142,  and  in 
the  girders  $110,000,  or  total  cost  of  iron  $196,410  (as  given 
in  report);  leaving  for  the  masonry  lining,  concrete  filling, 
and  sinking  of  the  wells  and  cylinders  $109,992,  or  per  linear 
foot  of  cylinder  $46.50.  This  last  calculation  is  made  by  the 
writer,  as  he  understands  the  data  given  above. 

87.  Also  for  the  end  piers  of  the  Kentucky  and  Indiana  canti¬ 
lever  bridge  across  the  Ohio  River  at  Louisville,  brick-lined 
cylinders  of  iron  were  sunk  and  subsequently  filled  with  con¬ 
crete,  constituting  a  similar  construction  to  the  above.  The 
pier  on  the  Indiana  side,  carrying  one  end  of  a  240-ft.  through- 
span,  was  composed  of  two  plate-iron  cylinders,  metal  thick¬ 
ness  f  in.,  in  sections,  riveted  together  as  the  work  progressed. 


FOUNDATIONS  FOR  HIGH  BUILDINGS. 


365 


The  brick  lining  rested  on  a  shoe  or  cutting  edge  at  the  bottom 
about  1  ft.  high  ;  the  thickness  of  the  brick  wall  was  18  ins.  for 
a  height  of  about  6  ft.;  the  thickness  was  then  reduced  to  13 
ins.,  and  continued  at  that  thickness  to  the  top.  The  cylinders 
were  sunk  simply  by  excavating  the  material  on  the  inside, 
and  building  the  cylinder  and  brick-work  at  the  same  time. 
After  reaching  the  proper  depth,  the  hollow  inclosed  space  was 
filled  with  concrete.  The  interior  diameters  of  the  cylinders  at 
the  bottom  were  10  ft.  ;  exterior  diameter,  13  ft.  of  in. ;  at  top 
interior,  7  ft.  6  in.  ;  exterior  diameter,  9  ft.  8f  in.  The  exca¬ 
vation  was  carried  down  in  the  rock  5  ft.  below  the  cutting 
edge.  The  following  is  the  cost  of  this  pier : 


2  cylinders  $-in.  iron,  105,662  lbs.  at  5.72  cts .  $6,043  §7 

504.0  cu.  yds.  earth  excavation  inside  of  cylinder  at  75  cts..  378  00 
144.2  “  rock  “  •“  “  $1.50..  216  30 

170.28  “  brick-work  in  lining  at  $11 .  1,873  °8 

299.75  “  concrete  filling  at  $6 .  1,798  50 

9.72  “  coping  at  $20 .  194  40 

6.5  “  cut  backing  at  $6.60 .  42  90 

Sundries .  78  59 

420.0  cu.  yds.  earth  filling  around  cylinders  at  25  cts .  105  00 

Total  for  two  cylinders . $10,630  64 


And  for  each  cylinder  $5315.32 ;  the  total  length  being  75.9  ft., 
or  per  linear  foot  of  caisson  $70.03.  The  engineer  in  charge, 
Capt.  C.  A.  Brady,  who  has  kindly  furnished  me  with  the 
above  and  the  following  data,  informed  me  that  the  rock  exca¬ 
vation  was  entirely  unnecessary,  as  it  was  a  hard,  compact 
slate,  which  would  reduce  the  cost  per  foot  of  length  to  $57.69. 

The  two  cylinders  for  the  pier  on  the  Kentucky  side  were 
larger,  carrying  the  projecting  arm  of  the  cantilever  span  260 
ft.  long,  as  they  also  acted  for  an  anchorage  pier.  Bottom 
diameter  15  ft.  5^  in.  exterior,  and  II  ft.  at  top,  lined  similarly 
to  the  above-described  cylinders;  total  length  112.2  ft.  ;  weight 
of  iron  in  two  cylinders  153,081  pounds.  The  unit  prices 
same  as  above  ;  quantities  considerably  increased.  Total  cost 
$18,498.36  for  the  two  cylinders,  for  each  $9249.18,  and  cost 
per  linear  foot  $83.32.  These  cylinder's  were  sunk  through 


Brick 


366 


A  PRACTICAL  TREATISE  ON  FOUNDATIONS. 


KENTUCKY  AND  INDIANA  BRIDGE 


Fig.  17. — Anchorage  Pier  for  Cantilever.  Cylinders  Settled  out  of  Line;  Adjusted 
to  Proper  Position  with  Inclined  Plates. 


Concrete 


FOUNDATIONS  FOR  HIGH  BUILDINGS. 


367 


28  ft.  clay,  5  ft.  of  gravel,  and  26  ft.  of  quicksand,  or  a  total 
depth  below  the  surface  of  63  ft.  Owing  to  caving  in  of  the 
quicksand,  letting  the  material  down  from  above,  the  cylinder 
was  thrown  out  of  position  ;  much  additional  labor  and  cost 
was  required  in  removing  material,  pumping  and  straightening 
cylinders,  adding  largely  to  the  cost  of  the  foundation  above 


Fig.  18. — Kentucky  and  Indiana  Bridge  Plan  of  Cylinders. 


an  ordinary  case.  The  cylinders  were  not  finally  straightened 
at  all.  To  bring  the  top  into  position  it  was  necessary  to 
rivet  a  slightly  inclined  elbow,  as  shown  is  Figs.  17  and  18.  De¬ 
tails  of  the  anchorage  connections  are  shown  in  Fig.  17,  near 
the  top.  These  last  cylinders  are  considerably  larger  than 
those  used  in  India,  but  the  difference  in  cost  is  also  very 
great.  Such  costs  may  be  almost  prohibitive  for  ordinary 
foundations  for  houses. 


SUPPLEMENT. 


THE  HAWARDEN  BRIDGE. 

88.  The  Hawarden  Bridge  over  the  river  Dee,  in  Eng¬ 
land,  consists  of  two  fixed  spans  120  ft.  each  and  of  one  draw- 
span  287  ft.  end  to  end.  The  clear  opening  on  one  side  of  the 
pivot  pier  is  140  ft.  and  on  the  other  87  ft.  It  is  a  double-track 
bridge,  with  a  (4)  four-foot-wide  footway  on  one  side.  The 
piers  are  brick  and  concrete  lined  cylinders.  The  wrought- 
iron  cylinder  for  the  pivot  pier  was  built  of  ^~in.  plates 
riveted  together,  having  a  special  rolled  section  of  iron,  form¬ 
ing  the  actual  cutting  edge,  inserted  between  them.  These 
plates  rise  vertically  for  a  height  of  9  ft.  outside  ;  but  inter¬ 
nally  they  slope  upward  toward  the  centre  of  the  cylinder, 
until  at  a  height  of  9  ft.  above  the  bottom  edge  the  internal 
diameter  is  30  ft.,  the  diameter  of  the  bottom  or  cutting  edge 
being  43  ft.,  the  reduction  being  6^  ft.  all  around,  forming  a  V- 
shaped  curb  or  section  9  ft.  high  and  ft.  wide  at  top. 
This  section  was  filled  with  cement  concrete  made  5  to  1  after 
being  sunk  on  the  bed  of  the  river.  Upon  this  bottom  section 
two  concentric  cylinders  made  of  iron  plate  were  built  to  a 
height  of  15  ft.,  making  a  total  of  24  ft.  in  height  above  the 
cutting  edge ;  this  was  sufficient  to  reach  from  the  bed  of  the 
river  to  above  high-tide  line.  The  cylinder  was  then  sunk  in 
its  proper  place,  about  200  tons  of  concrete  being  placed  in 
the  annular  space  between  the  two  plate-iron  cylinders.  The 
width  of  this  annular  space  was  reduced  to  about  5  ft.  above 
the  V-shoe  or  section,  and  in  it,  resting  on  the  concrete,  a 
cylindrical  brick  wall  5  ft.  thick  was  built,  the  internal  diameter 

369 


3/0 


SUPPLEMENT. 


being  30  ft.  and  the  external  diameter  40  ft.  This  brick  wall  was 
carried  well  up  above  the  water  surface.  Dredging  was  then 
commenced  inside  the  cylinder,  using  the  clam-shell  dredge. 
As  the  material  was  removed  and  the  brick  walls  were  built  up 
above  the  iron  casing,  the  cylinder  gradually  sank.  Except  that 
the  iron  plating  was  only  carried  to  the  height  of  24  ft.  above 
the  cutting  edge, — the  hollow  brick  cylinder  having  no  sheathing 
above  that  point  outside  nor  inside,  and  that  the  brick  walls 
were  5  ft.  in  thickness  and  diameter  40  ft.  out  to  out  of  wall, 
— the  general  construction  was  the  same  as  described  for  the 
cylinders  of  the  K.  and  I.  bridge  described  in  paragraph  87, 
Part  III. 

When  the  excavating  and  sinking  commenced  the  weight 
on  the  cutting  edge  was  about  6  tons  per  linear  foot.  The 
cylinder  was  sunk  to  a  depth  of  48  ft.  below  the  bed  of  the 
river.  With  a  weight  of  about  2300  tons  the  caisson  could  be 
controlled  easily  by  careful  and  skilful  handling  of  the  clam¬ 
shell  dredge,  aided  by  pumping  water  under  the  cutting  edge. 
The  cylinder  was  sunk  mainly  through  sand,  but  if  it  rested  at 
any  point  on  bowlders  or  lumps  of  hard  clay  or  other  material, 
it  was  found  easy  to  remove  these  by  pumping  water  under 
them.  This  latter  process  was  also  found  greatly  to  reduce  the 
frictional  resistance  on  the  surface  of  the  iron  or  brick  work. 
This  same  effect  was  referred  to  in  discussing  pneumatic  caissons 
as  due  to  the  escape  of  the  compressed  air  under  the  caissons 
and  rising  along  its  outer  surface.  After  reaching  the  proper 
depth  below  the  bed  of  the  river,  concrete  was  lowered  in 
“  pigeon-trap”  boxes  through  the  water  inside  of  the  brick 
cylinder  and  deposited  at  the  bottom  ;  this  was  continued  un¬ 
til  its  depth  was  about  18  ft.  The  concrete  was  made  with 
strong  cement,  in  the  proportions  of  5  to  1.  When  this  mass 
of  concrete  had  set,  it  was  found  practicable  to  pump  the 
water  out  of  the  cylinder,  and  the  rest  of  the  concreting  was 
deposited  in  the  dry.  This  concrete  was  mixed  6  to  1,  and 
filled  the  cylinder  to  a  height  of  about  65  ft.  above  the  bottom ; 
then  a  floor  of  brick-work  in  cement  was  laid  over  the  whole 
surface,  on  top  of  which  was  placed  a  large  granite  block,  9  ft. 


SUPPLEMENT. 


37* 


square  and  3  ft.  thick,  for  the  central  pivot-bearing,  and  also 
the  masonry  for  the  circular  track.  This  is  probably  the  largest 
cylinder  ever  sunk  under  the  brick-well  system  so  common  in 
India.  The  cement  mortar  used  in  the  brick-work  seems  to 
have  been  mixed  in  the  proportions  of  1^  to  I  for  the  lower 
portion  and  3  to  1  for  the  upper  portion. 

For  the  other  piers  two  cylinders  were  used,  each  cylinder 
being  about  12  or  14  ft.  outside  diameter  and  6  or  8  ft.  inside 
diameter,  allowing  brick  walls  of  about  3  ft.  thick.  These  were 
sunk  to  the  proper  depth  and  filled  with  concrete,  as  in  the 
pivot  piers.  Three  cylinders  were  used  in  one  of  the  piers. 
Brick  arches  were  then  built,  connecting  the  cylinders  at  the 
tops. 

The  necessary  piles  for  fenders  and  other  purposes  were 
sunk  by  aid  of  water  jets.  A  2-in.  pipe  was  temporarily  spiked 
to  the  side  of  the  pile,  which  was  then  placed  in  position  ;  the 
pipe  was  then  connected  to  the  steam-pump  by  a  flexible  hose. 
The  water  forced  through  it  discharging  at  the  foot  of  the  pile, 
caused  it  to  sink  with  any  desired  degree  of  rapidity.  During 
the  operation  of  sinking  the  pile  was  easily  moved  or  turned 
into  any  position.  When  the  pile  had  reached  the  proper 
depth  the  pump  was  stopped,  pipe  and  hose  removed,  and 
then  fastened  to  another  pile.  By  this  method  piles  were  sunk 
in  sand  to  the  depth  of  20  to  25  ft.  in  two  minutes,  “without 
the  delay,  uncertainty,  or  damage  which  so  frequently  accom¬ 
pany  the  ordinary  system  of  pile-driving.  Sometimes  a  nodule 
of  clay  or  erratic  bowlder  of  the  glacial  drift  was  encountered 
by  the  pile,  but  by  sinking  another  pipe  down  to  the  under  side 
of  the  stone  or  nodule,  and  pumping,  the  obstruction  sinks 
away  in  advance  of  the  pile,  which  rapidly  follows.  Within  a 
half  of  an  hour  of  the  pumping  being  stopped  the  sand  settles 
around  the  pile,  and  no  amount  of  ordinary  pile-driving  will 
stir  it  a  fraction  of  an  inch.” 

The  total  cost  of  this  bridge  was  $355,000, — equivalent  to 
$545  per  linear  foot.  No  division  of  this  amount  between  the 
substructure  and  superstructure  is  made  in  the  description  from 
which  the  above  is  taken. 


372 


SUPPLEMENT. 


89.  FOUNDATIONS  AND  FLOORS  FOR  THE  BUILDINGS  OF 
THE  WORLD’S  COLUMBIAN  EXPOSITION; 

On  the  subject  of  foundations  for  the  buildings  of  the 
World’s  Fair  at  Chicago,  Mr.  A.  Gottlieb  has  made  an  interest¬ 
ing  report,  which  was  published  in  the  Engineering  News,  from 
which  the  following  facts  are  taken. 

Chicago  Subsoil. — Commencing  at  the  surface  and  proceed¬ 
ing  downward,  the  following  lay  and  thickness  of  strata  are 
recorded  in  some  of  the  many  soundings  made,  among  which 
there  were  considerable  variations :  Upper  surface  black  soil, 
then  sand  5  to  8  ft.;  quicksand  4  to  10  ft.;  soft  clay  6  to  10  ft.; 
soft  blue  clay  6  to  10  ft.;  blue  clay  ;  hard  blue  clay  ;  hard-pan.. 
Average  depth  to  hard-pan  26  to  36  ft.  below  surface. 

The  sand,  when  loaded  with  2\  tons  per  square  foot,  settled 
•f  in.,  and  very  uniformly,  and  after  this  there  was  no  further 
appreciable  settlement.  On  sand  filling  over  mud  holes  a  load 
of  1^  tons  per  square  foot  settled  from  1  to  3  ft., and  kept  sink¬ 
ing  with  the  continuance  of  the  load  on  the  platform.  In  such 
places  piles  were  driven.  Whereas  on  the  regular  bed  of  sand 
a  load  of  about  1^  tons  per  square  foot  was  allowed.  The  plat¬ 
forms  supporting  the  columns  of  the  structures  were  constructed 
of  several  layers  of  plank  and  solid  timber  scantlings,  having 
sufficient  top  dimensions  for  the  bottoms  of  the  columns  to 
rest  upon  easily,  and  spreading  outwards  and  downwards,  so  that 
in  all  cases  the  area  of  the  bottom  of  the  platforms  should  be 
great  enough  to  limit  the  pressure  to  1^  tons  per  square  foot  of 
surface.  He  also  gave  some  interesting  experiments  on  the 
resistance  of  timber  columns  under  compression,  as  well  as  the 
resistance  to  crushing  of  timber  across  the  grain,  recommend¬ 
ing  the  safe  unit  loads  to  be  used.  These  were  not,  however, 
materially  different  from  the  safe  loads  already  given  in  this 
volume. 

MAXIMUM  AIR-PRESSURE  IN  PNEUMATIC  CAISSONS. 

90.  It  has  been  stated  that  the  limit  of  depth  in  the  pneu¬ 
matic  process  has  been  generally  accepted  as  100  ft.  below  the 


SUPPLEMENT. 


373 


water  surface.  The  writer  has  also  ventured  to  express  the 
opinion  that,  with  due  care  and  reasonable  precautions  in  select¬ 
ing  men  and  providing  for  their  comfort  and  health,  greater 
depths  could  be  reached  with  safety.  We  have  also  seen 
that  in  the  St.  Louis  and  Memphis  bridges  the  depth  or 
immersion  below  the  water  surface  reached  108  or  109  ft. 
Many  lives  were  lost  in  the  sinking  of  the  St.  Louis  caissons. 
No  report  has  been  made  on  the  other  in  regard  to  this  point, 
so  far  as  I  have  been  able  to  find  out.  In  the  Engineering 
News  of  March  16,  1893,  it  is  stated  that  a  tunnel  is  now  being 
constructed  8  X  10  ft.  in  cross-section,  by  the  East  River  Gas 
Company,  which  will  be  half  a  mile  long  when  completed. 
The  headings  of  this  tunnel  are  now  500  ft.  out  from  the  New 
York  side  and  100  ft.  out  from  the  Long  Island  end.  The 
men  have  worked  in  compressed  air  at  the  respective  depths  of 
134  ft.  and  147  ft.  below  mean  low  tide.  The  men  work  in 
four-hour  shifts.  Thus  far,  one  foreman  has  died  and  three 
workmen  have  been  brought  out  unconscious.  It  is  not  stated 
what  precautions,  if  any,  have  been  taken  for  the  health  and 
comfort  of  the  men. 

Even  as  matters  stand,  the  percentage  of  death  and  paralysis 
or  unconsciousness  would  not  seem  to  compare  unfavorably 
with  that  in  many  preceding  caissons  when  sunk  to  much  less 
depths  and  requiring  less  intensity  of  air-pressure. 

SOUNDINGS  AND  BORINGS. 

91.  The  importance  of  accurate  and  thorough  soundings 
or  borings,  in  order  to  determine  the  character  and  lay  of  the 
strata  underlying  the  bed  of  the  river  at  any  bridge  site,  has 
been  earnestly  urged  in  the  first  part  of  this  volume.  (See  Art. 
33,  paragraphs  25,  26,  27.)  In  the  Engineering  News  of  April 
1 3,  1893,  is  found  a  very  instructive  description  of  the  removing 
and  subsequent  rebuilding  of  a  pivot  pier  of  a  bridge  over 
the  Coosa  River  at  Gadsden,  Ala.,  which  had  badly  settled 
on  one  side,  “the  pivot  going  down  stream  about  7  ft.  and 
nearly  throwing  the  swing-span  into  the  river.  The  accident 
occurred  at  an  extraordinarily  high  stage  of  water,  the  river 


374 


SUPPLEMENT. 


being  subject  to  a  rise  of  nearly  40  ft.  at  this  point.”  From 
the  description  and  the  accompanying  drawings,  it  seems  that 
there  was  a  depth  of  12  ft.  of  water  at  ordinary  low  stages, 
and  4  ft.  of  gravel,  overlying  the  solid  rock.  The  layer  of 
gravel,  it  is  stated,  was  “  so  compact,  indeed,  that  it  had 
proved  impenetrable  to  the  sounding-rod  of  the  engineer 
originally  in  charge  of  the  bridge,  and  was  supposed  by  him  to 
be  solid  rock.”  From  this  statement  we  are  led  to  infer  that  the 
sounding  was  made  with  an  ordinary  straight  rod,  which  was 
struck  with  a  hammer  or  maul  in  order  to  determine  the  nature 
of  the  material  at  the  bed  of  the  river.  That  it  is  difficult,  if 
not  impracticable,  to  drive  an  inch  or  an  inch  and  a  half  rod 
to  any  great  depth  in  a  compact  bed  of  gravel  or  even  sand 
is  readily  admitted  ;  but  that  it  would  not  penetrate  at  all,  but 
would  give  a  rebound  and  a  sound  easily  recognized  when  a  rod 
is  simply  lifted  and  dropped  on  solid  rock,  cannot  be  so  readily 
admitted.  If  only  one  sounding  was  taken,  the  rod  may  have 
rested  on  a  bowlder  of  large  size ;  several  soundings,  however, 
would  certainly  have  determined  the  question.  It  was  cer¬ 
tainly  unwise  to  have  sunk  an  open  caisson,  to  simply  rest  on 
the  bed  of  a  river  subject  to  such  sudden  and  great  floods, 
without  a  thorough  examination  of  the  nature  of  the  bed. 
The  driving  of  a  single  pile  would  have  settled  the  doubt  ;  and 
again,  it  would  seldom  be  found  that  the  bed  of  a  river,  if  solid 
rock,  would  be  sufficiently  level  to  sink  a  solid  bottom  caisson 
on  it  without  some  careening.  The  bed  being  practically 
horizontal,  should  have  at  least  aroused  a  suspicion  that  the 
bed  was  either  gravel,  sand,  or  clay.  At  the  depth  given — only 
twelve  feet  of  water — it  would  seem  that  an  ordinary  coffer-dam 
should  have  been  used,  in  which  event  the  treacherous  nature 
of  the  bed  would  have  been  discovered,  a  good  foundation 
secured,  and  the  subsequent  danger  and  cost  avoided.  Had 
the  soundings  been  made  by  the  use  of  pipes  and  a  force-pump 
the  existence  of  the  gravel  bed  would  have  been  determined. 
(See  Art.  33,  paragraphs  25,  26,  27;  and  also  Plate  IV,  Fig.  10.) 

The  height  of  the  pier  was  about  80  ft.,  and  it  contained 
1 100  cu.  yds.  of  masonry.  As  the  pier  had  to  be  removed 


SUPPLEMENT. 


375 


entirely,  a  large  coffer-dam  had  to  be  built  around  the  pier. 
“  This  dam  gave  a  great  deal  of  trouble  during  the  prosecution 
of  the  work,  owing  to  the  porosity  of  the  gravel  and  to  the  irreg¬ 
ularity  of  the  surface  of  the  rock  upon  which  the  gravel  lay , 
although  three  separate  rows  of  sheet-piling  were  driven  through 
it  to  the  rock  and  well  puddled  between.”  Where  sheet-piling 
could  be  so  easily  driven,  an  iron  rod  ought  to  have  penetrated. 
The  subsequent  cost  due  to  this  error  is  not  given  ;  but  the 
following  estimate  will  be  below  rather  than  above  the  actual 
cost,  and  will,  I  hope,  have  the  effect  of  strongly  impressing 
upon  young  engineers  the  importance  of  obtaining  reliable  in¬ 
formation,  especially  when  it  can  be  obtained  at  a  cost  of  only 
$100  to  $200  at  the  outside: 


iioo  cu.  yds.  masonry  @  $8= . $8,8oo  oo 

“  “  “  “  lifted  from  the  pier,  landed,  cleaned,  and 

piled,  @  $3, . 3,300  00 

“  rehandled  and  relaid  @  $3, . 3,300  00 

Constructing,  pumping  out  of  coffer-dam,  and  excavation . 5, 000  00 

Constructing  false-work,  trusses,  etc.,  to  support  the  draw-span,  .  5,000  00 

(Including  in  the  above  all  material,  labor,  necessary  plant,  etc.) 

Total  cost, . $25,400  co 


The  writer  has  made  no  estimates  of  actual  quantities  and 
costs,  and  hopes  that  he  has  not  overestimated  the  costs.  He 
hopes  also  that  the  engineer  of  this  work  will  not  consider  the 
above  as  intended  for  a  criticism  of  his  skill  or  ability.  We  all 
make  blunders,  and  the  writer  has  recorded  his  own  in  several 
places  in  this  volume  ;  and  he  has  taken  every  occasion  to 
describe  failures  and  blunders,  with  no  other  object  in  view  than 
that  of  recording  faithfully  all  facts  exactly  as  they  have  oc¬ 
curred,  as  he  felt  it  his  duty  to  do  ;  and  he  further  believes  that 
more  useful  knowledge  and  experience  can  be  obtained  from  a 
study  of  blunders  and  failures  than  from  successes. 

The  writer  had  a  somewhat  similar  experience  when  build¬ 
ing  a  bridge  across  the  Warrior  River  in  Alabama.  He  sounded 
with  a  solid-pointed  rod,  and  reported  rock  at  5  ft.  below  the 
bed  of  the  river,  covered  with  sand  and  gravel.  This  result 


376 


SUPPLEMENT. 


was  somewhat  anticipated,  as  the  rock  upon  which  stood  an 
adjacent  pier  ioo  ft.  from  the  one  under  examination  waswell 
exposed  at  low-water.  There  were  only  some  6  or  8  ft.  of  water 
at  lower  stages  at  the  site  of  the  pier.  He  put  in  a  coffer-dam, 
however,  which  developed  the  fact  that  there  was  at  least  15 
ft.  of  sand  over  the  rock  ;  and  having  pumped  the  water  out 
and  having  excavated  about  10  ft.,  it  was  then  found  necessary 
to  drive  piles  in  the  bottom  of  the  excavation  to  a  further 
depth  of  15  to  20  ft.  in  order  to  obtain  a  safe  resistance. 

One  of  our  most  eminent  Southern  engineers  constructed 
a  bridge  across  the  Big  Sandy  River,  W.  Va.,  on  a  bed  of 
bowlders  and  gravel  at  no  very  great  depth  below  the  bed  of 
the  river,  which  stood  for  many  years  without  showing  any 
settling,  but  after  more  than  ten  years  of  constant  use  one  of 
these  piers  settled  out  of  line.  The  writer  called  upon  him 
for  advice  as  to  the  suitableness  of  a  bed  of  clay,  under  the 
Ohio  River  at  Point  Pleasant,  W.  Va.,  for  supporting  a  high 
and  heavy  pier  with  its  superstructure  and  load.  His  reply 
was,  that  when  younger  he  thought  that  he  knew  a  good 
foundation  bed  when  he  saw  it,  but  after  the  settling  of  the 
Big  Sandy  pier  he  had  come  to  the  conclusion  that  he  knew 
but  little  of  this  subject.  Long  experience  will  generally  make 
us  more  cautious,  and  therefore  safer  advisers. 

The  engineer  who  has  never  blundered  and  never  met  with 
any  failures  has  had  but  little  experience  or  has  acquired  but 
little  useful  information,  unless  he  is  wise  enough  to  profit  by 
the  experience  of  others,  and  has  put  himself  to  the  trouble  of 
acquainting  himself  with  their  failures. 

THE  ACTUAL  RESISTANCE  OF  BEARING-PILES. 

92.  The  importance  of  the  subject  of  the  bearing  power  of 
piles  cannot  be  overestimated,  and  any  information  on  the 
subject  is  valuable  and  instructive,  and  no  excuse  is  needed 
for  again  referring  to  it.  The  following  tables  and  remarks 
are  taken  from  the  columns  of  the  Engineering  News  of  Feb- 


SUPPLEMENT. 


3  77 


ruary  23,  1893.  Outside  of  the  valuable  statistics  given,  the 
object  of  the  article  seems  to  be  to  prove  that  the  formula 


Safe  Load  = 


2  wh 

s+  i’ 


(I) 


in  which  w  —  weight  of  hammer  in  tons  or  pounds  (the  safe 
load  being  in  the  same  unit)  and  h  —  its  fall  in  feet,  s  -  set 
under  last  blow  in  inches,  will  give  safe  and  reliable  working 
loads.  On  this  point  the  editor  says: 

“No  formula  can  attempt  to  state  exactly  how  much  should 
be  spent  in  such  a  case,  or  how  much  load  can  safely  be  placed 
on  the  pile.  What  the  Engineering  News  formula  does  pur¬ 
port  to  do  is  to  set  a  definite  limit,  high  enough  for  all  ordi¬ 
nary  economic  requirements,  up  to  which  there  is  no  record  of 
pile  failures,  excepting  one  or  two  dubious  cases,  where  a  hid¬ 
den  stratum  of  bad  material  lay  beneath  the  pile,  and  above 
which  there  are  instances  of  both  excess  and  failure,  with  an 
increasing  proportion  of  failures  as  the  limit  is  exceeded. 

“  If  it  does  this,  as  it  is  believed  to  do,  it  is  in  all  cases  a  safe 
guide,  having  the  risk  of  semi-fluid  material  existing  beneath 
the  foot  of  the  pile,  and  in  most  cases  a  sufficient  guide  as  well. 
But  when  a  large  number  of  piles  are  to  be  driven,  or  extra 
heavy  loads  are  to  be  sustained,  ordinary  prudence  would  dic¬ 
tate  the  ascertaining  by  experiment  just  what  the  piles  will 
bear,  or,  if  failure  would  do  no  great  harm,  taking  chances  with 
greater  loads  without  experiment,  under  favorable  conditions. 
The  formula  is  not  intended  to  be  rigidly  applied  to  such 
eases  as  this.”  The  above  statement  really  contains  all  that  the 
writer  of  this  volume  has  contended  for,  when  discussing  the 
value  of  pile-driving  formulae. 

The  comparative  merits  of  the  Enginee?ing  News  formula, 
Trautwine’s  formula,  Crowell’s  modified  formula,  Sanders’ 
formula,  have  been  the  subject  of  much  discussion,  heated  and 
even  acrimonious  ;  and  as  the  writer  has  taken  little  part  in  the 
battle  of  the  formulae,  and  is  going  to  recommend  the  Engineer¬ 
ing  News  formula,  if  any  is  necessary  or  used,  no  comparisons 


37^ 


SUPPLEMENT. 


will  be  made,  and  the  formulae  results  given  below  refer  only 
to  this  latter  formula  in  the  following  records. 

TABULATED  RECORDS. 

Table  I. 


Locality  and  Soil. 

Hammer,  Fall  and  Set. 

Actual 

Load. 

Safe  Load  by 
Eng.  News 
Formula. 

Chestnut  Street  Bridge. .  . 

Mud . 

.  ..  .  f  in. 

40,300 

29,100  lbs. 

Neuilly  Bridge . 

Gravel. . . 

105,300 

19,700  “ 

Hull  Docks . 

45,000 

Mud . 

to  56,000 

24,000  “ 

Royal  Border  Bridge.  .  .  . 

Sand  and  gravel . 

156,800 

53,700  “ 

Phila.  Experiments . 

14,560 

Soft  mud . . 

to  20,120 

6,060  “ 

U.  S.  test-pile . 

Silt  and  clay  . 

59,600 

6,600  “ 

French  rule . 

.  .  .  1,344  lbs.,  4  ft. 

No  set. 

56,000 

10,742  “ 

“  The  Engineering  News  formula  gives  the  closest 

approxi- 

mation  of  the  three ;  and  secondly,  this  formula  for  ultimate 
load  (—  six  times  the  safe  load)  is  not  intended  to  be  used  for 
determining  ultimate  loads,  and  not  alleged  to  give  them  with 
any  accuracy,  for  the  reason  that  the  ultimate  load  is  a  much 
more  variable  quantity  than  the  permanent  safe  load.  At 
least  we  so  understand  it  ;  and  certainly  the  safe  load  is  the 
only  thing  we  are  aiming  to  determine  or  care  to  know.” 

“It  should  be  borne  in  mind  that  in  some  if  not  all  of 
these  cases  the  surrounding  soil  bears  an  unknown  proportion 
of  the  load,  so  that  the  load  actually  coming  on  the  piles  may 
be  several  times  less  than  stated.” 

Table  II. 

SUMMARY  OF  INSTANCES  OF  BEARING  POWER  OF  PILES 
GIVEN  IN  MORE  DETAIL  BELOW. 

Case  No.  A«**™™*  ^^ZV*  Factor-of-Safety.  Material. 


1  .  13.333  1,684  7.9  Mud 

2  .  14,560  6,067  2.4  “ 

3...  ...  22,400  23,450  —  1.0  “ 

4 .  44,800  28,333  x.6  “ 


SUPPLEMENT.  379 


Case  No. 

Actual  Ultimate 
Load. 

Safe  Load  by 
Formula. 

Factor-of-Safety. 

Material. 

(  I5U75  ) 

, 

1  r-3  ) 

5 . 

1  to  47,375  f 

11,400 

i  to  4. 1  1 

Alluvial 

6 . 

59,618 

6,741 

8.8 

Mixed 

7 . 

75,000 

44/080 

t-7 

Sand 

8 . 

224,000 

112,000 

2.0 

“ 

t  8,020 

i-7 

Sandy 

13,440 

1  9,520 

1.4 

Mud 

10 . 

|  6,400  + 

<  10,183 

Not  determinable. 

“ 

(  to  13,300  + 

(  to  20,000 

i  ( 

i  ( 

II . 

(  6,400  + 

(  10,667 

t 1 

tt 

(  to  13,333  + 

1  to  11,790 

a 

it 

12 . 

over  22,400  + 

37,500 

a 

it 

TESTS  OF  PILES  IN  WHICH  THE  RESISTANCE  TO  EXTRACTION 
IS  THE  ONLY  EVIDENCE  AS  TO  ULTIMATE  BEARING  POWER. 


Case  No. 

Actual  Ultimate 
Load. 

Safe  Load  by 
Formula. 

Factor-of-Safety. 

Material. 

13 . 

490 

j 

108 

Sandy 

i  to 

1,288 

1  to 

251 

Clay 

14 . 

15,850 

25,067 

Unknown 

15 . 

i. 

25,000 

50,000 

5,000 

5,000 

Rotten  rock 

< 

50,000 

j 

51,250  ) 

Sand 

16 . 

i  to 

83,000 

\  to 

O 

O 

<D 

CO 

00 

it 

17 . 

\ 

71,300 

j 

12,800  1 

Clay 

l  to 

71,300 

\  to 

17,920  f 

A  detailed  account  of  experiments  for  case  No.  1  above 
has  been  given  already.  See  Art.  42,  par.  84. 

Case  2.  Philadelphia,  1873.  Soft  river  mud.  Trial  pile 
loaded  with  14,560  lbs.  five  hours  after  driving,  and  sank  but  a 
very  small  fraction  of  an  inch.  Under  20,160  lbs.  it  sank  f  in.; 
under  33,600  lbs.,  sank  5  ft.  (Records,  Table  I,  show  hammer 
weight  1600  lbs.,  falling  36  ft.;  penetration  or  set  18  in.  Safe 
load  by  formula,  6067  lbs.) 

Case  3.  Mississippi  River  at  East  St.  Louis,  1868-69. 
Soft  muddy  bottom,  with  5  or  6  ft.  of  water.  Piles  in  tempo¬ 
rary  railway  trestle  of  three-pile  bent,  15  ft.  from  centre  to 
centre,  driven  about  20  ft.  Penetration,  2\  to  3  in.  The  piles 
settled  badly  in  a  very  short  time  under  locomotives  weighing 


38° 


SUPPLEMENT . 


not  over  30  tons,  so  that  the  load  on  a  pile  could  hardly  have 
exceeded  22,400  lbs.  (Safe  load  by  formula, — taking  27J  ft. 
mean  fall,  2\  ins.  mean  penetration,  1600  lbs.  hammer, — 23,450 
lbs.  Many  of  the  data  of  this  case  are  quite  dubious,  especially 
the  weight  given  for  locomotives.  There  were  very  few,  if  any, 
in  Missouri  in  1868-69  so  light  as  30  tons.  It  is  more  likely 
the  load  on  each  pile  was  double  that  stated.) 

Case  4.  Perth  Amboy,  1873.  Pretty  fair  mud,  30  ft.  deep. 
Four  piles,  12,  14,  1 5 ,  and  18  ins.  diameter  at  top,  6  to  8  in.  at 
foot,  were  driven  in  a  square  to  depths  of  from  33  to  35  ft. 
Distance  apart  not  given.  A  platform  was  built  upon  the 
heads  of  the  piles  and  loaded  with  179,200  lbs.,  say  44,800  lbs. 
per  pile.  After  a  few  days  the  load  was  removed.  The  18-in. 
pile  had  not  moved;  the  12-in.  pile  had  settled  3  in.,  and  the 
14  and  15  in.  piles  had  settled  to  a  less  extent. 

(Hammer  1700  lbs.,  falling  25  ft.,  with  2-in.  penetration. 
Safe  load  by  formula,  28,333  lbs.) 

Case  5.  The  record  of  driving  uncertain  and  unintelligible. 

Case  6.  Proctorsville,  La.  Material :  mud,  sand,  and  clay  ; 
wet.  Trial  pile  (driven  alone)  said  to  have  been  30  ft.  long, 
yet  it  is  said  to  have  sunk  5  ft.  4  in.  by  its  own  weight,  and  to 
have  been  driven  29  ft.  6  in.  deeper,  making  34  ft.  10  in. 
driven  length;  cross-section  12^  X  12  ins.  at  top,  and  ii£  X  11 
ins.  sharpened  to  4  ins.  square  at  foot.  Weight,  161 1  lbs.  Head 
capped.  Pile  bore  59,618  lbs.  without  settlement,  but  settled 
slowly  under  62,500  lbs.  Fall  during  last  ten  blows  regulated 
to  5  ft.  exactly.  Penetration  last  ten  blows  ranged  from  ^  to 
in.;  mean,  0.35  in.;  last  blow,  f  in. 

(Hammer,  910  lbs. ;  safe  load  by  formula,  6741  lbs.,  being 
very  far  below  what  the  pile  actually  sustained.  This  is 
another  case  of  those  piles  in  soft  material  whose  resistance  is 
not  fairly  measured  by  the  blows  given  when  first  driving,  but 
can  only  be  fairly  gauged  by  trying  blows  after  the  mud  has 
had  time  to  set.) 

Case  7.  Buffalo.  Material :  wet  sand  and  gravel.  Piles 
driven  in  nests  of  from  9  to  13  piles.  Test-pile  of  beech,  20  ft. 


SUPPLEMENT. 


381 


long  after  being  driven  and  cut  off.  Driven  length  20  ft.,  3  ft. 
in  stiff  clay;  cross-section,  15  ins.  diameter  at  top.  A  load  of 
50,000  lbs.  remained  on  the  pile  for  27  hours,  but  produced  no 
appreciable  effect.  The  load  was  increased  20,000  lbs.  at  a 
time,  and  left  for  24  hours  after  each  increase.  A  gradual 
settlement,  aggregating  f  in.,  took  place  under  75,000  lbs.,  and 
the  pile  then  came  to  rest.  The  total  settlement  increased  to 
1^  ins.  under  100,000  lbs.,  and  to  3-^  ins.  under  150,000  lbs. 
During  the  experiments  the  ground  was  kept  in  a  tremor  by 
the  action  of  three  pile-drivers  at  work  on  the  foundations. 
Subsequent  use  shows  that  74,000  lbs.  is  a  safe  load. 

(Hammer,  1900  lbs.  ;  fall,  29  ft.;  set,  1.5  in.  Safe  load  by 
formula,  44,080  lbs.) 

Case  8.  As  a  result  of  the  tests  in  Brooklyn,  N.  Y.  Ma¬ 
terial :  wet,  loamy,  micaceous  quartz  sand,  becoming  quicksand 
wherever  it  was  much  trodden.  As  the  result  of  the  tests  it 
was  believed  that  for  a  pile  driven  33  ft.  into  the  earth  to  the 
point  of  ultimate  resistance,  with  a  ram  weighing  2240  lbs. 
and  falling  30  ft.  at  the  last  blow,  the  extreme  supporting 
power  due  to  frictional  surface  was  224,000  lbs.,  or  I  ton  per 
superficial  foot  of  the  area  of  its  circumference. 

(Safe  load  by  formula  for  0.0  and  0.2  in.  penetration, 
134,000  to  112,000  lbs.) 

Case  9.  Material :  sand,  with  some  mud.  Piles  seem  to 
have  been  driven  either  by  a  steam  pile-driver  delivering  60 
blows  per  minute,  ram  weighing  2205  lbs.,  falling  30  in.,  or 
by  an  ordinary  hand  engine,  ram  992  lbs.,  fall  6  ft.  7  in. 
Penetration  at  last  blow,  £  in.  to  in.  If  the  penetration 
was  over  f  in.  on  the  average  of  the  last  100  blows,  the  rule 
was  to  put  in  extra  piles.  The  load  seems  to  have  been  about 
13,440  lbs.  per  pile. 

(Safe  load  by  formula  for  2205-lb.  hammer,  8020  lbs.;  with 
the  992-lb.  hammer,  safe  load  9520  lbs.) 

Case  10  belongs  to  the  same  set  of  experiments  as  Case  1, 
and  is  found  fully  described  in  part  first  of  this  volume. 

Case  11  also  belongs  to  Case  1.  Safe  load  by  formula, 
10,667  to  1 1,790  lbs. 


382 


SUPPLEMENT. 


Case  12.  Lake  Pontchartrain  Trestle,  La.  About  6  miles 
of  trestle  crossed  the  lake  proper,  and  the  remainder  (16  miles) 
crossed  the  adjoining  sea-swamp.  Four-pile  bents  15  ft.  be¬ 
tween  centres.  Material  of  swamp  :  several  feet  of  soft,  black 
vegetable  mould,  lying  upon  soft  clay,  with  occasional  strata 
of  sand  1  to  2  ft.  thick.  Piles  sank  from  5  to  8  ft.  of  their  own 
weight,  and  then  about  as  much  more  with  hammer  (about 
2500  lbs.)  resting  on  head  of  pile.  Two  piles  65  ft.  long  were 
driven,  one  on  the  top  of  the  other,  and  penetrated  9  in.  with 
over  100  ft.  driven ;  but  a  30-foot  fall,  30  minutes  after  driving 
a  pile,  gave  only  3  in.  penetration.  Piles  65  to  75  ft.  long  ; 
weight  of  ram  about  2500  lbs.  ;  fall  about  30  ft.  ;  penetration 
3  to  12  in.  “  No  settlement  has  been  observed  in  the  entire 
length  of  the  structure  to  date.”  With  four  piles  in  a  bent, 
and  the  bents  1 5  ft.  centres,  the  load  on  each  pile  probably 
has  not  exceeded  22,400  lbs. 

(Safe  loads  for  3,  6,  9,  and  12  ins.  penetration,  37,500, 
21,430,  15,000,  and  11,538  lbs.  by  formula.  “As  the  only 
proper  fall  to  be  considered  in  a  case  like  this  is  the  3-in.  pene¬ 
tration,  which  occurred  after  30  minutes’  intermission,  the 
check  here  is  excellent.”) 

The  remaining  cases  in  Table  2,  viz.,  13,  14,  15,  16,  and  17, 
only  are  important  as  enabling  inferences  to  be  drawn  as  to 
the  bearing  power  of  piles  from  their  resistances  to  being 
pulled  out  after  being  driven.  In  Cases  13  and  14  the  piles 
were  lifted  by  the  action  of  ice,  and  the  load  was  estimated  by 
the  uplifting  force  of  ice,  which  was  taken  at  18,850  lbs.  per 
pile.  “  The  case,  in  fact,  is  one  of  exceptionable  doubtfulness 
in  all  respects.” 

Case  15.  Material:  wet,  decomposed  mica  schist.  A 
wrought-iron  pipe,  3^  ins.  outside  diameter,  3  ins.  inside,  was 
inserted  in  a  bore-hole  6  in.  in  diameter  and  30  ft.  deep,  and 
driven  14  ft.  to  rock.  After  several  hours’  work  with  block 
and  fall,  the  pipe  was  pulled  in  two  by  using  two  hydraulic 
jacks  of  unequal  power,  one  on  each  side,  by  means  of  which 
the  pipe  had  been  raised  8  ins.  The  fracture  took  place  in 


SUPPLEMENT. 


383 


the  thread,  where  the  wall  thickness  was  reduced  from  ^  to  ^ 
in.,  leaving  a  cross-sectional  wall  area  of  3^  X  3. 1416  X  i  = 
I.23  sq.  ins. 

Since  the  pipe  had  been  slightly  raised  under  the  pull 
which  soon  after  caused  its  rupture,  the  latter  was  evidently 
about  equal  to  the  resistance  of  the  pipe  to  being  withdrawn. 
We  can  estimate  at  the  amount  of  this  pull  by  estimating  the 
tensile  strength  of  the  iron  at  the  point  of  rupture. 

The  conditions  of  the  experiment  were  very  crude,  but 
considering  that  the  pipe  was  no  doubt  weakened  by  canting 
from  side  to  side  under  the  unequal  forces  exerted  by  the  two 
jacks,  as  well  as  by  repeated  blows  in  driving  and  repeated 
tensile  strains  in  drawing,  and  by  the  removal  of  the  outer 
shell  of  the  iron,  in  cutting  the  thread,  the  tensile  strength 
could  hardly  have  exceeded  40,000  lbs.  per  square  inch,  and, 
on  the  other  hand,  it  was  perhaps  not  less  than  20,000,  giving 
25,000  to  50,000  lbs.  as  the  extreme  load. 

(Hammer  of  350  lbs.  falling  8  ft.,  set  •§■  in.,  sustained  25,000 
to  50,000  lbs.  Safe  load  by  formula,  5000  lbs.  scant.) 

Case  16.  Pensacola,  Fla.  Material:  clean,  hard,  sharp, 
white  quartz  sand.  All  the  sand  would  pass  through  a  sieve 
having  openings  ^  in.  square.  Water  filtered  through  it  came 
out  perfectly  clear.  One  cubic  foot  of  it  would  hold  6  qts.  of 
water.  The  2-ton  hammers  could  only  drive  it  about  20  ft. 
The  water  was  about  1  i-J  ft.  deep.  Seven  piles,  selected  as 
representing  the  average  of  all,  were  tested  with  upward  pulls 
of  20,000  lbs.  each  without  moving,  and  one  of  these  was  after¬ 
wards  tested  with  upward  pulls  sufficient  to  cause  motion  (as 
recorded  below),  and  finally  withdrawn.  This  pile  was  29  ft. 
long,  16  ft.  in  sand,  including  its  point — 2  ft.  long.  One  foot 
of  this  length  was  in  loose  sand,  which  had  been  excavated 
and  had  fallen  back.  The  average  diameter  of  the  part  in  the 
sand  was  13^-  ins.,  including  the  bark.  Weight  of  pile  1632  lbs. 
Pile  tested  two  months  after  driving.  As  neither  the  weight 
nor  fall  of  hammer  nor  set  are  given,  the  formula  can  only  be 
applied  by  assuming  the  necessary  data. 


384 


SUPPLEMENT. 


Safe  Load  by  Formula. 

1st . W  —  2,200  lbs.,  k  —  30  ft.,  j- =  0.5,  88,000  lbs. 

2d..... . W—  4,100  “  h  —  33  “  j  =  o.o,  270,000“ 

3d . .IV  =4,100  “  h—  10“  j=o.6,  51,250“ 

The  tests  on  the  trial  pile  resulted  as  follows : 

78,000  lbs . No  movement. 

80,000  “  . ....Resisted  %  min.  and  then  rose  very  slowly.  Rose  24 

ins.  in  30  minutes. 

82,000  “  . if  minutes. 

83,000  “  . . .  T  minute.  Rose  2-J-  ins.  in  all  in  30  minutes. 

60,000  “  . 18  hours.  No  movement. 

64,000  | . Rose  3  ins.  in  one  hour,  6  ins.  in  all. 

74,000  “  ) 

50,000  “  . For  two  days.  No  movement. 


The  very  small  loads  obtained  by  the  tests  in  this  case  seem 
to  confirm  the  view  already  expressed,  that  the  resistance  of  a 
pile  to  an  upward  pull  must  be  less  than  that  to  a  downward 
pressure,  especially  in  a  pure  sand,  exerting  relatively  a  small 
resistance  to  being  broken  up,  but  offering  a  great  resistance  to 
a  downward  pressure. 

Case  1 7.  Albert  Dock,  Hull,  England.  Material:  compact 
bluish  clay,  above  which  there  were  from  3  to  5  ft.  of  peat,  and 
above  this  silt  and  sand  in  places.  Piles  of  ordinary  rough 
Memel  bark  timber,  from  10  X  12  to  14  X  15  his. ;  average  12^- 
ins.  square,  from  20  to  40  ft.  long.  Driven  length  from  6  to 
30  ft. ;  average  18^-  ft.  Most  of  the  piles  were  driven  from  10 
to  20  ft.  into  the  clay,  and  were  nearly  or  quite  in  that  material 
alone ;  but  a  few  of  the  shorter  piles,  driven  in  a  sloping  side 
of  the  dock,  were  entirely  in  the  silt,  while  a  few  others  entered 
the  peat  without  reaching  the  clay.  The  piles  were  driven 
close  together  in  two  rows  5  ft.  apart  forming  a  coffer-dam,  the 
space  between  the  two  rows  having  been  filled  with  puddled 
clay  to  above  high-water  mark.  Before  the  piles  were  with¬ 
drawn  the  puddle  was  removed  down  to  a  level  rather  under 
high-water  mark  of  ordinary  neap  tides.  Weight  of  ram  2240 
lbs.,  height  of  fall  varied  from  5  to  6  ft.,  and  the  penetration 
from  0.5  to  0.75  in.  Four  hundred  and  twenty  piles  were 
withdrawn  and  300  observations  recorded.  The  force  was 
applied  by  men  working  a  winch  and  estimated  by  testing  that 


SUPPLEMENT. 


3SS 


of  the  men  in  lifting  known  weights.  The  piles  were  drawn 
consecutively,  so  that  one  side  of  each  pile  was  nearly  or  quite 
free  from  frictional  contact,  the  opposite  one  was  in  loose  con¬ 
tact  with  the  adjoining  pile,  and  only  the  remaining  two  sides 
resisted  by  friction  with  the  ground. 

The  average  total  force  required  to  draw  a  pile  was  75,869 
lbs.  The  author  deducts  from  this  2340  lbs.  (=  12  X  12  in. 
X  15  lbs.  per  square  inch)  for  suction  and  2240  lbs.  for  weight 
of  pile,  leaving,  say,  71,300  lbs.  as  the  frictional  resistance  to 
drawing  the  pile. 

(Safe  load  by  formula  with  f-in.  set  after  5-ft.  fall,  12,800 
lbs.;  with  J-in.  set  after  6-ft.  fall,  17,920  lbs.) 

The  writer  regards  the  above  records  as  the  most  complete 
and  satisfactory  of  any  heretofore  published,  and  although 
there  is  a  noticeable  want  of  uniformity  and  consistency  in 
many  respects,  much  can  be  learned  from  a  careful  study  of 
all  of  the  facts.  The  methods  of  testing  are  suggestive,  and 
the  importance  of  accurate  records  are  clearly  set  forth,  and 
whereas  there  are  great  differences  between  the  actual  loads 
and  the  loads  obtained  by  the  formula,  a  comparison  of  these 
would  seem  to  lead  to  the  conclusion  that  what  is  claimed  by 
the  authors  of  the  formula,  viz.,  that  the  formulae  will  usually 
err  on  the  safe  side,  can  be  reasonably  admitted. 

As  was  mentioned  in  another  place,  it  is  but  reasonable  to 
suppose  that  it  would  require  a  smaller  force  to  pull  a  pile  than 
it  would  to  cause  it  to  sink ;  as  in  the  first  case  when  the  pile 
is  lifted  a  gradually  decreasing  surface  is  in  contact  with  the 
surrounding  soil,  and  in  fact  in  the  stiffer  soils  it  is  only  neces¬ 
sary  to  move  the  pile  but  a  few  feet  upward  before  the  fric¬ 
tional  resistance  will  disappear  almost  entirely  if  the  lifting  force 
is  accurately  in  the  prolongation  of  the  axis  of  the  pile  ;  whereas 
in  sinking  a  pile  by  direct  pressure,  gradually  increasing  sur¬ 
faces  are  presented  in  contact  with  the  soil,  so  that  not  only 
has  frictional  resistance  to  be  overcome,  but  a  certain  amount 
of  lateral  and  downward  compression  of  the  material  has  to 
be  effected  by  the  required  displacement  of  the  material. 
This  explains  the  fact  that  structures  founded  on  piles  often 


386 


SUPPLEMENT 


settle  a  few  inches,  and  then  remain  fixed  for  a  time,  when  a 
small  additional  settlement  may  take  place. 

PRESERVATION  OF  TIMBER — VULCANIZED  TIMBER. 

93.  The  great  importance  of  increasing  the  durability  of 
timber,  and  of  devising  some  means  by  which  many  kinds  of 
timber  which  are  now  considered  useless  can  be  utilized,  has 
been  realized  by  a  few  engineers  and  builders,  but  has  not 
received  as  much  recognition  as  the  subject  demands.  There¬ 
fore  the  writer  wishes  to  give  some  prominence  to  a  process 
which  is  not  generally  known,  by  which  it  is  claimed  all  woods 
are  increased  in  strength  and  stiffness,  are  made  more  dura¬ 
ble,  and  that  many  kinds  of  wood  now  considered  worthless 
can  be  utilized  in  building  any  kind  of  timber  structures. 
Several  methods  have  been  given  in  the  preceding  pages  of 
preserving  timber  (see  Article  38).  It  has  not,  I  believe,  been 
claimed  that  any  of  these  methods  increased  the  strength  of 
timber,  or  even  that  to  any  extent  it  rendered  the  weaker  and 
inferior  grades  suitable  for  the  ordinary  structures. 

The  method  now  to  be  described  seems  to  the  writer  to 
be  rational,  simple,  and  economical,  and  if  the  results  thus  far 
obtained  are  confirmed  by  future  experiments,  and  the  pro¬ 
cesses  can  be  economically  carried  out,  it  would  seem  that  the 
question  of  timber  preservation  will  have  been  satisfactorily 
solved.  An  interesting  description  of  the  method  of  produc¬ 
ing  so-called  “  vulcanized  timber  ”  will  be  found  in  the  Electn- 
cal  World ,  March  11,  1893.  From  this  magazine  the  following 
points  of  interest  are  mainly  taken. 

Wood  as  it  occurs  in  nature  consists  of  cellulose  impreg¬ 
nated  with  resin,  volatile  oils,  sugar,  gum,  tannin,  protein 
bodies,  and  the  usual  mineral  constituents  of  plants.  When 
wood  is  heated,  as  in  ordinary  distillation,  the  cellulose  decom¬ 
poses  and  a  chemical  change  takes  place  between  it  and  the 
natural  constituents  of  the  sap,  resulting  in  a  most  powerful 
antiseptic  mixture  containing  acetic  acid,  methyl  alcohol, 
acetone,  methyl  acetate,  tarry  matter  containing  phenol, 
creosote,  carbolic  acid,  and  about  thirty  other  chemicals  of 


SUPPLEMENT. 


387 


lesser  practical  importance.  These  chemicals  and  antiseptics 
result  from  the  action  of  heat  on  the  natural  sap  of  the  wood, 
and  are  entirely  different  from  the  original  sap,  which  allows 
the  attacks  of  microscopic  fungi  and  decay.  If  timber  is 
heated  to  the  temperature  which  will  produce  the  above 
change  and  the  antiseptic  mixture  is  kept  in  it  by  pressure, 
instead  of  distilling  it  out,  experiments  show  that  the  change 
will  produce  a  stronger  and  more  durable  timber  than  it  was 
originally.  “  Wood-vulcanizing  is  heating  wood  and  timber 
under  great  pressure.”  The  wood  is  heated  in  closed  cylin¬ 
ders  from  eight  to  twelve  hours,  at  a  temperature  ranging  from 
300°  to  500°  Fahr.,  while  under  a  pressure  of  150  to  200  lbs. 
per  square  inch.  A  circulation  of  superheated  and  dried 
compressed  air  removes  the  surface  moisture  and  any  water 
that  does  not  take  part  in  the  reaction,  and  combine  with  the 
woody  constituents.  Cylinders  of  steel  105  X  ft.  in  length 
and  diameter,  respectively,  are  employed.  The  timbers  to  be 
treated  can  be  loaded  on  cars  or  trucks  and  run  direct  into  the 
cylinders.  When  subjected  to  the  proper  temperature  and 
pressure  during  the  necessary  interval  of  time,  the  timber  is 
removed.  “  This  apparently  makes  decay  impossible  by  seal¬ 
ing  up  the  pores  with  antiseptic  matter,  which  becomes  solid 
on  cooling.  The  changed  sap  is  very  dark  or  black.” 

“  The  process  of  vulcanizing  seasons  all  timber,  preventing 
any  further  warping,  checking,  or  cracking.  Such  timber  is 
not  influenced  by  atmospheric  agencies,  bacteria  or  spores,  and 
requires  no  paint  for  protection.  The  albuminous  constituents 
of  the  natural  wood  have  been  coagulated  by  the  high  heating, 
and  rendered  insoluble.” 

These  are  seemingly  extravagant  claims,  which  should  only 
be  accepted  when  fully  established  by  careful  and  accurate 
experiments  in  sufficient  numbers  and  after  a  sufficient  lapse 
of  time — at  least  so  far  as  durability  is  concerned.  But  when 
so  established  they  should  be  accepted  by  engineers,  whether 
in  conflict  with  theories  or  not,  and  even  when  seeming  to  be 
unreasonable.  Some  chemists  have  stated  to  the  writer  that 
it  seems  probable  that  the  chemical  changes  could  take 
place  only  when  such  a  temperature  was  reached  that  would 


SUPPLEMENT. 


388 

char  and  destroy  the  woody  fibre  ;  so  theoretically  the  results 
seem  doubtful.  But  they  at  least  seem  reasonable  in  view  of 
the  facts:  1st.  That  timber  is  seasoned  artificially  by  being 
subjected  to  hot  air  for  comparatively  short  periods  of  time. 
2d.  That  timber  is  rendered  more  durable  and  capable  of  re¬ 
sisting  the  attacks  of  the  teredo  when  the  moisture  has  been 
removed  and  the  pores  refilled  with  some  at  least  of  the 
above-mentioned  antiseptic  compounds  by  pressure.  3d.  It 
would  seem  to  be  more  economical  to  convert  the  fluids  found 
in  the  timber  into  antiseptics  and  keep  these  in  the  timber  by 
similar  processes  to  those  adopted  when  creosote  is  forced 
under  pressure  into  the  timber,  the  creosote  being  first  ob¬ 
tained  by  distillation  from  other  and  different  timber. 

However,  to  sustain  the  claims  of  the  inventors  or  users  of 
the  vulcanized  process  for  preserving  timbers,  they  offer  the 
following  results  of  experiments  and  experience.  And  when 
we  remember  the  fact  that  the  annual  consumption  of  timber 
in  this  country,  equals  twice  the  amount  of  material  supplied 
by  the  annual  growth  of  our  forests,  it  is  to  be  hoped  that  an 
efficient  and  economical  means  of  preserving,  strengthening, 
and  hardening  timber  has  been  discovered. 

Experiments  made  on  a  number  of  specimens  show  that 
the  strength  is  increased  as  much  as  18.78  per  cent,  and 
amount  of  deflection  is  decreased  by  13  per  cent,  as  compared 
with  specimens  of  the  same  timber  that  have  not  been  treated. 

Timber  not  treated  by  the  vulcanizing  process,  but  painted, 
showed  a  loss  of  38.12  per  cent  in  strength  as  compared  with 
the  treated  timber,  after  a  lapse  of  some  considerable  time  in 
exposed  situations.  Frames  made  partly  of  the  vulcanized 
timber  and  partly  of  the  timber  in  its  natural  state,  after  the 
lapse  of  eight  years  or  more  were  found  sound  and  solid  so 
far  as  the  prepared  timber  was  concerned,  but  those  parts 
formed  of  the  natural  timber  had  almost  entirely  rotted.  Mr. 
Tracy,  late  Secretary  of  the  Navy,  made  a  thorough  investiga¬ 
tion  of  the  subject,  and  recommended  that  the  vulcanized 
timber  should  be  used  in  certain  parts  of  the  ships  being  con¬ 
structed  for  the  Government.  These  facts  would  seem  to 
justify,  in  part  at  least,  the  claims  of  the  manufacturers. 


TENSILE  STRENGTH  OF  CEMENTS. 


389 


TENSILE  STRENGTH  OF  CEMENTS. 

REQUIREMENTS. 

Tensile  Strength  required 
in  Pounds  per  square  inch. 
One  day.  One  week. 


Portland  cement,  neat. . .  * . no  300 

“  “  1  to  2  mortar .  100 


Fineness:  80  per  cent  must  pass  through  a  sieve  of  10,000  meshes. 


Average  Tensile  Strength  in  Pounds. 


rime  of  set  in  water . 

•  |  day. 

I 

week. 

1 

mon. 

1 

year. 

2 

years. 

3 

years. 

4 

years, 

Portland,  Burham  (neat) . 

■  167 

429 

615 

798 

700 

764 

782 

“  1  to  2  mortar. . . 

.  ... 

141 

258 

468 

532 

632 

658 

“  1  to  3  “ 

169 

224 

404 

520 

552 

. .  . 

Giant  (neat) . 

348 

422 

682 

694 

736 

771 

“  1  to  2  mortar  . . . 

166 

280 

490 

564 

680 

674 

1  to  3  “  . . . 

140 

234 

420 

512 

572 

.  . . 

Rosendale,  Union  (neat) . . 

240 

228 

510 

542 

650 

654 

“  1  to  2  mortar.  .  . 

34 

94 

394 

430 

514 

522 

A  natural  Portland  cement  from  Maria  Island,  Tasmania,  weighs  113  lbs.  per 
bushel.  Specific  gravity  3.152;  slow  setting. 

7  days.  14  days.  28  days. 

Tensile  strength  (neat) .  536  536  646 

“  “  1  to  3  mortar .  150  ...  258 

The  above  is  taken  from  Eng.  News,  April  13,  1893.  It 
is  of  special  interest,  as  the  tables  contain  the  longest  time  tests 
of  which  we  have  any  records,  and,  extending  through  a  period 
of  four  years,  give  the  gradual  increase  in  hardness  and  strength 
with  increase  of  age.  The  fall  in  strength  between  the  one- 
and  two-year  test  of  the  Burham  neat  cement  is  due  either  to 
a  typographical  error  or  an  error  in  the  record ;  the  one-year 
test  should  no  doubt  be  698  lbs.  The  results  are,  however, 
recorded  as  found  in  the  columns  of  the  News.  There  is  also 
apparently  an  error  in  the  one-week  test  of  same  cement  in  the 
I  and  2  or  the  1  and  3  mortar. 

These  tables  are  given,  as  interesting  and  instructive  results 
of  experiments,  without  comment. 


390 


SUPPLEMENT. 


EXTRACTS  FROM  THE  BUILDING  ORDINANCES  OF  THE  CITY 

OF  CHICAGO. 

Class  ist.  Buildings  devoted  to  the  sale,  storage,  or  manu¬ 
facture  of  merchandise,  and  stables. 

Class  2d.  Residences  for  three  or  more  families,  hotels, 
boarding  or  lodging  houses  occupied  by  twenty-five  or  more 
persons,  and  office  buildings. 

Class  3d.  Residences  for  one  or  two  families,  or  for  less 
than  twenty-five  persons. 

Class  4th.  Buildings  used  as  assembly  halls  for  large  gather¬ 
ings  of  people. 

Fire-proof  construction  applies  to  buildings  in  which  all 
parts  that  carry  weights,  stairs,  elevator  enclosures  and  their 
contents  are  made  of  incombustible  material,  and  in  which  all 
metallic  structural  members  are  protected  against  the  effects  of 
fire  by  coverings  of  an  incombustible  and  slow  heat-conducting 
material,  such  as  brick,  hollow  tiles  or  burnt  clay,  porous  terra¬ 
cotta,  and  two  layers  of  plastering  on  metal  lath. 

In  “  skeleton  construction  ”  all  external  and  internal  loads 
and  strains  are  transmitted  from  the  top  of  the  building  to  the 
foundations  by  a  skeleton  or  framework  of  metal.  In  columns 
of  rolled  iron  or  steel,  the  different  parts  shall  be  riveted  to 
each  other,  and  shall  be  united  by  riveted  connections  to  the 
beams  and  girders  resting  upon  them.  In  cast-iron  columns, 
each  successive  column  shall  be  bolted  to  the  one  below  it  by 
at  least  three  f-in.  bolts,  and  the  beams  and  girders  shall  be 
bolted  to  the  columns. 

Slow-burning  construction  applies  to  all  buildings  in  which 
the  structural  members  are  made  wholly  or  in  part  of  com¬ 
bustible  material,  but  throughout  which  all  materials  shall  be 
protected  against  injury  from  fire  by  coverings  of  incombusti¬ 
ble,  slow  heat-conducting  materials,  such  as  above  described. 
Oak  posts  of  greater  sectional  area  than  100  sq.  in.  need  not 
have  any  special  fireproof  covering. 

“  Mill  construction  ”  applies  to  buildings  in  which  all  of  the 
girders  and  joists  supporting  floors  and  roofs  have  a  sectional 


SUPPLEMENT. 


391 


area  of  not  less  than  72  sq.  in.,  and  above  the  joists  of  which 
there  is  laid  a  solid  timber  floor  not  less  than  3f  in.  thick. 
Wooden  posts  in  buildings  of  this  class  are  to  have  an  area  of 
at  least  100  sq.  in.  Iron  columns,  girders,  or  beams  must  be 
protected  as  provided  for  fire-proof  buildings,  but  the  wooden 
posts,  girders,  and  joists  need  not  be  covered. 

Ordinary  construction  applies  to  those  buildings  in  which 
the  timber  and  iron  structural  parts  are  not  protected  with 
fire-resisting  coverings. 

Buildings  of  classes  1  to  3  shall  be  made  entirely  of  fire¬ 
proof  construction,  if  100  feet  or  more  in  height,  and  entirely  of 
slow-burning  or  of  mill  construction  if  between  60  and  100  ft. 
in  height.  They  may  be  built  of  ordinary  construction  if  under 
60  ft.  in  height.  Buildings  of  class  4,  containing  not  more  than 
600  seats,  may  be  built  of  ordinary  construction  ;  if  containing 
from  600  to  1500  seats,  they  shall  be  built  of  slow-burning  or 
of  mill  construction,  and  if  more  than  1500  seats,  of  fire-proof 
construction  entirely.  If  movable  scenery  is  used,  they  must 
be  of  slow-burning  or  mill  construction  when  containing  less 
than  1000  seats,  and  entirely  fire-proof  if  containing  more  than 
1000  seats. 

No  building  shall  be  erected  of  greater  height  than  130  ft. 
from  the  sidewalk  level  to  the  highest  point  of  external  bearing 
walls.  The  height  of  no  building  of  skeleton  construction  shall 
be  more  than  three  times,  and  no  isolated  building  of  masonry 
construction  more  than  fourtimes,  its  least  horizontal  dimension. 

Loads. — Buildings  of  class  1  shall  be  designed  for  a  mini¬ 
mum  load  of  150  lbs.  for  each  square  foot  of  floor  surface.  For 
buildings  of  the  classes  2  and  4,  a  live  load  of  70  lbs.  per  square 
foot  shall  be  assumed  in  addition  to  all  permanent  loads.  In 
determining  the  strength  of  posts  and  the  area  of  foundation 
for  many  storied  buildings  of  classes  2,  3,  and  4,  allowances  are 
to  be  made  for  the  fact  that  the  before-mentioned  live  load  of 
70  lbs.  per  square  foot  is  but  an  occasional  load,  which  rarely 
occurs  simultaneously  upon  corresponding  parts  of  many  floors, 
and,  if  so,  for  a  brief  period  only. 

Foundations  shall  be  constructed  of  either  of  the  following: 


392 


SUPPLEMENT. 


Cement,  concrete,  dimension  or  rubble  stones,  sewer  or  paving- 
bricks,  timber  piles  covered  with  a  grillage  of  oak  timber,  or  an 
oak  grillage  may  be  used,  all  timber  to  be  below  city  datum. 
Rails  or  beams  used  as  parts  of  foundations  must  be  thoroughly 
imbedded  in  concrete,  and  around  the  exposed  external  sur¬ 
faces  of  such  concrete  foundations  there  must  be  a  coating  of 
cement  mortar  at  least  I  in.  thick.  In  pile  foundations  the 
piles  shall  be  driven  to  reach  the  underlying  stratum  of  hard 
clay  or  rock,  and  shall  not  be  loaded  with  more  than  25  tons. 
In  other  but  pile  foundations  the  limits  of  loads  for  different 
kinds  of  soil  are  to  be  the  following; 

If  the  soil  is  a  layer  of  pure  clay  not  less  than  15  ft.  thick, 
without  admixture  of  any  foreign  substances,  excepting  gravel, 
3500  lbs.  per  square  foot.  If  the  soil  is  a  layer  of  dry  sand, 
15  ft.  or  more  in  thickness  and  without  admixture  of  clay, 
loam,  or  other  foreign  substance,  4000  lbs.  per  square  foot. 

If  the  soil  is  a  mixture  of  clay  and  sand,  3000  lbs.  per  sq.  ft. 

The  offsets  of  foundations  of  concrete  alone  shall  not  exceed 
one  half  the  height  of  the  respective  courses,  and  such  con¬ 
crete  foundations  must  not  be  loaded  more  than  8000  lbs.  per 
square  foot.  If  reinforced  by  rails  or  beams,  the  offsets  must 
be  so  adjusted  that  the  fibre  strain  per  square  inch  shall  not 
exceed  12,000  lbs.  for  iron  nor  16,000  lbs.  for  steel. 

Dimension  stones  must  have  uniform  beds,  and  offsets  of 
layers  must  not  be  more  than  three  quarters  of  the  height  of 
the  individual  stones.  The  load  per  square  foot  in  foundation 
piers  of  dimension  stones  shall  not  be  more  than  10,000  lbs. 

In  brick  piers  there  shall  be  at  every  offset  a  bond-stone  at 
least  8  in.  thick,  and  at  the  top  of  each  pier  a  cap-stone  at 
least  10  in.  thick,  or  in  all  such  cases  a  bond-plate  of  cast  or 
rolled  iron.  The  extreme  loads  on  brick-work  laid  in  mortar 
of  any  cement,  established  as  a  standard  by  the  Society  of 
Civil  Engineers  of  the  Northwest,  are  25,000  lbs.  per  squai  e  foot, 
and  18,000  lbs.  for  brick-work  laid  in  ordinary  cement  mortar. 
The  use  of  soft  bricks  for  piers  is  prohibited. 

Maximum  Permissible  Stresses.— Cast-iron  crushing  stress  for 
plates,  15,000  lbs.  per  square  inch;  for  lintels,  brackets,  or 


SUPPLEMENT. 


393 


corbels,  compression  13,500  lbs.  per  square  inch  and  tension 
3000  lbs.  per  square  inch  ;  for  girders,  beams,  corbels,  brackets, 
and  trusses  per  square  inch,  16,000  lbs.  for  steel  and  12,000 
lbs.  for  iron. 

For  plate  girders : 

max.  bending  moment  in  ft.-lbs. 

Flange  area  =  - yry. - , 


in  which  D  —  distance  between  centres  of  gravity  of  flanges  in 
feet ; 

C  —  13,500  for  steel  and  10,000  for  iron. 


Web  area  = 


max.  shear 

C 


where  C  —  10,000  for  steel  and  6000  for  iron. 

For  rivets  in  single  shear  per  square  inch  of  rivet  area : 

If  shop  driven,  9000  lbs.  for  steel  and  7500  lbs.  for  iron. 
“  field  “  7500  “  “  “  “  6000  “  “  “ 

Maximum  Permissible  Load. 

For  cast-iron  round  columns  : 

a  —  area  of  column  in  square  inches ; 

/  —  length  of  column  in  inches ; 
d  —  diameter  “  “  “ 

For  cast-iron  rectangular  columns: 

„  10000. a  (  ,  .  , 

5  =  - -y —  J  l  and  a  as  above  ; 

1  +7. - id—  least  horizontal  dimension  of  column. 

1  Sood  [ 

For  riveted  or  other  forms  of  wrought-iron  columns: 

1 2000 . a 

S  =  - - 7—-  • 

1  36000F 


J  /  and  a  as  before  ; 
j  r  —  least  radius  of  gyration  in  inches. 


10000. a  ( 
1  ^  600 d 2  l 


394 


SUPPLEMENT. 


For  riveted  or  other  steel  columns  if  more  than  60 r  in 
length  : 

6oA 

17000  —  — Jtf. 

For  riveted  or  other  steel  columns  if  less  than  6or  in  length : 


5  =  13500.  a. 

I,  r,  and  a  as  in  preceding  examples. 
For  timber  girders: 


6'  = 


cbcP 

~T’ 


l—  length  of  beam  in  feet; 
b  —  breadth  of  “  “  inches; 

d=  depth  “  “  “  “ 

c  —  160  for  long-leaf  yellow  pine  ; 
=  120  for  oak ; 

=  100  for  white  or  Norway  pine. 


area  of  post  in  square  inches ; 
least  side  of  rectangular  post  in  inches  ; 
length  of  post  in  inches. 

600  for  white  or  Norway  pine  ; 

800  for  oak ; 

900  for  long-leaf  yellow  pine. 

Wind-bracing.— In  all  buildings  the  height  of  which  is 
more  than  one  and  one  half  times  their  least  horizontal  dimen¬ 
sion,  allowances  shall  be  made  for  a  wind-pressure  of  not  less 
than  30  lbs.  for  each  square  foot  of  exposed  suiface.  As 
factors  of  resistance  to  wind-pressure  may  be  counted: 
1st.  Dead  weight  of  structure,  especially  in  its  lower  parts; 
2d.  Diagonal  braces;  3d-  Rigidity  of  construction  between 
vertical  and  horizontal  members;  4th.  Construction  of  iron  or 
steel  columns  in  such  manner  as  to  pass  through  two  stories 
with  joints  breaking  in  alternate  stories. 


For  timber  posts : 


5  = 


ac 


1  + 


250<r/3 


a  = 
d  = 
/  = 
c  - 


SUPPLEMENT. 


395 


Towers,  domes,  and  spires  may  be  built  on  the  top  of  the 
roofs  of  buildings  in  classes  i,  2,  and  3,  but  shall  not  occupy 
more  than  one  quarter  of  the  street  frontage  of  any  building 
or  have  a  base  area  of  more  than  1600  square  feet.  If  built  to 
heights  between  60  and  90  feet  above  the  sidewalk,  they  shall 
be  of  slow-burning  construction,  and  if  more  than  90  feet  above 
sidewalk  of  fire-proof  construction.  Where  the  area  of  such 
spire,  dome,  or  tower  exceeds  100  square  feet  its  supports  shall 
be  carried  down  to  the  ground  and  shall  be  of  slow-burning 
construction  if  the  supported  structure  is  between  60  and  90  feet 
high,  and  of  fire-proof  construction  if  more  than  90  feet  high. 

SOME  RECENT  CONTRACT  PRICES  ON  RAILWAY  MASONRY. 


For 

rock-faced  ashlar  masonry. .  . 

. . .  $8  00  per 

cubic  yard. 

U 

broken  range  masonry . 

6.00 

U 

U 

U 

u 

random  rubble  “  . 

..  4.50 

( i 

u 

ii 

u 

ashlar  arch  “  . 

u 

a 

u 

a 

rubble  “  “  . 

u 

u 

u 

u 

rectangular  culvert  masonry. 

. ..  3.00 

u 

u 

u 

u 

paving,  dry . 

..  1.50 

a 

a 

a 

u 

“  ,  cemented . 

a 

u 

u 

a 

vertical  wall . 

..  2.50 

u 

a 

u 

u 

slope  “ . 

u 

u 

u 

u 

riprap . 

u 

u 

u 

n 

white-oak  timber . 

per 

1000  feet  B.  M 

These  would  seem  to  be  bottom  prices  for  the  several  classes 
of  work,  and  are  those  actually  paid  on  railway  work  in  1892. 


EXPERIMENT  ON  THE  BEARING  POWER  OF  PILES. 

(Taken  from  Eng.  News ,  July  6,  1893.) 

As  it  was  desired  to  erect  the  Chicago  Public  Library 
building  on  a  pile  foundation  constructed  by  driving  three  rows 
of  piles  in  a  trench,  and  to  determine  whether  it  would  be  safe 
to  load  each  pile  with  30  tons,  the  following  experiment  was 
made  on  a  group  of  four  of  the  piles  ;  and  in  order  to  make  the 


SUPPLEMENT. 


3^ 

experiment  under  the  same  conditions  as  would  exist  under 
the  structure,  three  rows  of  piles  were  driven  into  the  trench, 
the  piles  in  the  middle  row  being  then  cut  off  below  the  level 
at  which  those  in  the  outside  rows  were  cut  off,  so  as  to  bring 
the  bearing  only  on  four  piles,  two  in  each  outside  row.  This 
gave  the  benefit  arising  from  the  consolidation  of  the  material 
by  the  other  piles.  Fifteen-inch  steel  I-beams  were  then  placed 
on  the  piles,  and  upon  these  a  platform  7  feet  by  7  feet  com¬ 
posed  of  12  X  12-inch  yellow-pine  timber. 

The  piles  were  driven  by  a  steam  hammer  of  the  Nasmyth 
type,  weight  4500  pounds,  fall  42  inches,  making  54  blows 
per  minute.  The  last  20  feet  were  driven  with  a  follower  of 
oak.  To  drive  the  last  foot  required  48  to  64  blows :  it  may  be 
estimated  that  without  using  the  follower  it  would  have  re¬ 
quired  24  to  32  blows  to  have  driven  it  the  distance  of  a  foot. 
In  the  same  soil  it  required  about  16  blows  of  a  drop-hammer 
weighing  3000  pounds  and  falling  30  feet  to  drive  the  last  foot 
with  a  follower,  and  32  to  36  blows  of  the  same  drop-hammer 
falling  15  feet  with  a  follower.  The  piles  were  driven  2\  feet 
between  centres,  three  in  a  row  along  the  trench.  They  were 
54  feet  long  and  were  driven  about  52^-  feet — about  27  feet  in 
soft,  plastic  clay,  23  feet  in  tough,  compact  clay,  and  2  feet  in 
hardpan.  They  had  an  average  diameter  of  13  inches,  circum¬ 
ference  of  41  inches,  and  area  at  small  end  of  80  square  inches. 
It  has  been  found  that  similar  piles,  after  being  driven  and 
allowed  to  stand  for  24  hours,  required  from  300  to  6co  blows 
of  the  above-described  hammer  to  drive  it  the  last  foot,  or  a 
repetition  of  300  to  600  blows  of  1 80, OOO  inch-pounds  each. 
The  heads  of  the  piles  were  sawed  off  27  feet  below  the  street 
grade,  the  lower  ends  being  about  80  feet  below  same.  The 
bearing  power  of  this  hardpan  by  Rankine’s  formula  may  be 
taken  at  170  pounds  per  square  inch,  and  by  empirical  results 
at  250  pounds  per  square  inch  ;  and  at  a  fair  assumption  it  may 
be  taken  at  200  pounds  per  square  inch.  The  extreme  average 
frictional  resistance  per  square  inch  of  sides  of  piles  like  those 
described,  as  deduced  by  experiments  made  under  analogous 
conditions,  may  be  taken  at  15  pounds  per  square  inch.  The 


SUPPLEMENT. 


397 


area  at  lower  end  beirg  80  square  inches,  and  the  bearing 
resistance  of  the  hard  pan  200  pounds  per  square  inch,  the 
extreme  point  resistance  will  be  16,000  pounds.  The  average 
total  exterior  surface  of  one  pile  will  be  about  25,000  (52  X  12  X 
41  =  25,584)  square  inches,  which  at  15  pounds  will  give  a  total 
frictional  resistance  of  375,000  pounds  or  an  ultimate  bearing 
power  of  195.5  tons,  or,  discarding  point  resistance,  of  about 
187  tons.  And  assuming  that  the  ultimate  crushing  strength  of 
wet  Norway  pine  is  not  over  1600  pounds  per  square  inch,  and 
with  a  safety  factor  of  3,  the  safe  load  will  not  be  over  533 
pounds  per  square  inch  ;  and  as  the  piles  have  a  minimum 
diameter  at  lower  end  of  8  inches  and  at  the  butt  16  inches, 
the  minimum  average  area  will  be  about  113  square  inches; 
hence  each  pile  should  not  have  to  carry  over  60,230  pounds, 
or  30  tons  about.  This,  then,  provides  a  factor  of  safety  of  3 
for  the  crushing  resistance  of  the  timber,  and  a  safety  factor 
of  6  for  the  frictional  resistance  of  the  soil.  If  the  timber  be 
loaded  to  one  half  of  its  ultimate  strength,  90,000  pounds  or  45 
net  tons  may  be  assigned  to  each  pile.  But  in  this  case  only 
30  tons  was  allowed.  The  experimental  test  was  made  by 
piling  pig-iron  on  the  platform  resting  on  four  of  the  piles, 
which  were  five  feet  centres,  the  entire  platform  being  7  7 

feet,  as  before  stated.  The  pig-iron  was  piled  up  at  irregular 
intervals.  When  four  feet  high  the  load  was  45,200  pounds, 
and  was  then  continued  until  at  the  end  of  about  four  days  it 
was  21  feet  high,  giving  a  load  of  224,500  pounds.  Levels  were 
taken,  but  no  settlement  had  occured.  By  the  end  of 
about  eleven  days  the  pile  of  iron  had  reached  the  height  of 
38  feet,  giving  a  load  of  404,800  pounds  upon  the  four  piles,  or 
about  50.7  tons  per  pile.  Levels  were  then  taken  at  intervals 
during  a  period  of  about  two  weeks,  and  no  settlement  having- 
been  observed,  a  load  of  30  tons  was  considered  perfectly  safe. 
The  iron  was  removed,  and  the  construction  of  the  building  was 
commenced. 


INDEX 


Abutments: 

arches  in . • 

for  arches . 

form  of,  wing,  U,  and  T 

designing  of . 

Arches: 

abutments  for . 


apron  walls  and  paving. 

brick,  used  in . 

“  construction  of . 

centres  for . 

construction  of . 

culverts . 

definition  of  terms  used  in.  .  . 

development  of  soffit . 

direction  of  pressure  in . 

hoop  iron  in . 

length  of . 

line  and  centre  of  pressure  in 

masonry  of . 

resultant  pressures . 

rolling  loads  on . 

rupture,  joint  of  . 

rules  in  construction  of . 

stability  of . 


stone  better  than  brick 
theory  of . 


skew . 

thickness  of  keystone 

tunnel . 

uses  of . . 

U  t* 


Art.  ro,  Par.  108;  Art.  14, 
“  24,  “  256  to  265 

“  10,  “  105;  Art.  11, 

. . .  Art.  10,  Par. 


. “  14.  “ 

Art.  24,  Par.  256  to  265 

.  Art.  12,  Par. 

.  “  16,  “ 

. Art.  16,  Par.  160, 

.  “  17,  “  185, 

Art.  12,  Par.  120;  Art.  17, 

.  Art.  24,  Par.  251, 

.  “  12,  “  123, 

.  “  13,  “  132, 

.  Art.  12,  Par. 

.  “  16,  “ 

.  “  12,  “ 


Art.  12,  Par.  118,  127, 

.  Art.  12,  Par. 

.  “  14,  “ 

.  “  12,  “ 

.  “  12,  “ 

.  “  16,  “ 

.  “  14,  “ 

.  “  16,  “ 

.  “  12,  “ 

.  “  17,  “ 

.  “  13,  “ 


16,  “ 

12,  “ 

16,  “ 


Par.  138 

Par.  1 14 
106 

138 

128 

159 

161,  162 
186 

Par.  185 
252,  253 
124,  125 
133.  134 
1 19 
163 
126 
1 16 

129,  130 

116 

136 

116,  117 
122 

167 

136,  137 

168 
US 

170-184 

131 

135 

164,  165 
121 

166 


399 


400 


INDEX. 


Bearing  resistances  of  materials: 
clay . 


Art.  i.  Par.  io 


clay  and  sand. 


conclusions .  Art.  i,  Par.  14,  15; 

determination  of..  Art.  i,  Par.  18;  Art. 42,  Par.  84,  85; 
gravel . Art.  1,  Par.  11,  14; 


sand. 


silt. 


tables  of. 


.  Art.  28,  Par.  306 ; 


tests  should  be  made. 
Brick: 

arches — sec  Arches. 


compressed.  . 
durability  of. 
making . 


sewer,  pavements. 


walls  of. 


“  56, 

“  76,  77 

1, 

“  13 

“  56, 

“  76 

“  56, 

“  80 

“  56, 

“  76 

29. 

■'  4 

“  56, 

“  76 

7;  also  see 

Supplement, 

Art.  42,  Par.  84,  85,  86 

“  x, 

“  II 

“  49. 

“  25,  29 

“  56, 

“  76,  77 

“  I> 

“  12 

“  50, 

-  46 

“  56, 

“  76 

“  I, 

“  16,  17 

“  56, 

“  76 

“  1, 

“  18 

Art.  15, 

Par.  150 

•  “  15, 

“  155 

“  15, 

“  149 

“  15, 

“  140,  I4T 

Par.  143, 

144,  145,  146 

Art.  15, 

Par.  148 

“  15, 

“  157,  158 

.  15,  Par. 

147,  152,  153 

15.  “ 

139,  142,  143 

15,  Par.  151 

15. 

16, 
*5, 


“  thickness  of . 

“  how  measured . 

when  to  be  used . 

Bridges  described: 

Cairo — 

construction,  method  of  sinking  and  depths  reached..  Art.  49,  Par.  29 

number  and  length  of  spans . 

East  River . 


156 

169 

154 


Hawkesbury . 
Memphis. . . . 
Morgan  City. 


49. 

«  * 

29 

49- 

«« 

24.  25 

53. 

<« 

65 

47, 

«  « 

6 

53. 

(< 

65 

49. 

(1 

27 

53. 

«< 

65 

47. 

t  i 

9 

Bridges  described  : 
Parkersburg. . 
Poughkeepsie. 


INDEX. 


401 


St.  Louis 


Susquehanna  and  Schuylkill — 
timber  caissons,  construction  of 

cribs  on  caissons . 

coffer-dams . 

Drawings . 

Caissons,  launching . 

advantages  in  design . 

accidents . 

sinking,  method  of . 

excavation  below  cutting  edge.  . 
number  and  length  of  spans . 


Tombigbee  River. 

accidents. . 

Point  Pleasant. .  . . 

Caissons: 

pneumatic,  defined 

air  in . 

air-lock . 


air  in,  effects  of . 

ti  tt  it  tt 

air  in,  reducing . 

ti  a  tt 

combined  with  open  crib 

concrete  in . 

construction  of  . .  .... 


depths,  limit  of . 

excavating  below  cutting  edge 

for  masonry  abutments . 

iron  used . 

machinery  for . 

pumps  for  mud . 

sinking,  method  of . 


shafts,  pipes,  valves 

signals . 

with  open  crib . 

Remarks . 


.  Art.  29,  Par.  4 

.  “  47,  “  5 

.  “  53,  “  65 

.  “  49,  “  26 

.  “  53,  “  65 

.  Art.  49,  Par.  29 i,  30,  31,  32 

.  “  49,  “  30,  36,  37,  38 

Art.  49,  Par.  31,  38;  Art.  50,  Par.  45 
.  .Plates  XIII,  XIV,  XV,  XVI,  XVII 

.  Art.  49,  Par.  33 

.  “  49,  “  34 

.  “  49,  “  35 

.  Art.  50,  Par.  39,  40,  41,  42 

.  Art.  50,  Par.  43 


* 1 

27,  “ 

279 

53,  “ 

65 

“ 

50,  “ 

46 

50,  “ 

47 

7,  Par.  286,  287,  288 

Art. 

47,  Par. 

1 

“ 

48,  “ 

11, 

12 

“ 

48,  “ 

13, 

15 

49,  “ 

26, 

29 

.  48, 

Par.  18, 

19, 

20 

Art. 

49,  Par. 

29 

‘  * 

48,  “ 

22 

“ 

50,  “ 

42 

.  Art.  51,  Par.  48,  49,  50,  51,  52 

.  Art.  50,  Par.  44 

.  Art.  49,  Par.  24,  25,  26,  27 

.  “  49,  “  28,  29,  2Q$,  32 

Art.  47,  Par.  3;  Art.  48,  Par.  20 

.  “  50,  “  43 

.  “  50,  “  45 

.  “  49,  “  28 

.  “  48,  “  23 

.  “  50,  “  41 

.  “  48,  “  16 

. Art.  50,  Par.  39,  40,  41,  42 

.  Art.  48,  Par.  13,  14,  17 

. Art.  48,  Par.  21 

.  “  49,  “  38 

.  .  “  5i,  “  53 


402 


INDEX. 


Caissons,  open: 

construction  of  . Art.  31,  Par.  20,  21,  22 

in  bed  of  streams .  Art.  31,  Par.  21;  Art.  45.  Par.  11S,  119,  120 

“  piles .  Art.  3i.  Par.  21 

..  ■<  . . .  Art.  45»  Par.  118,  119,  120 

when  used . . .  Art.  31,  Par.  22 

Caisson  and  open  crib  combined: 

construction  and  design  of . . .  Art.  51,  Par.  48,  49 

sinking,  method  of .  "  51*  “  5°.  51.  52 

uses  of .  Art.  47.  Par.  8;  Art.  51,  Par.  50,  51,  52 

Remarks .  Art.  51.  Par.  53 

Cement : 

hydraulic,  defined . . 

brands  of . 

effect  of  temperature  on . 

heavy  slow  setting .  . 

light  quick  “  .  . * . 

Portland . 

set  of .  Art.  19 


Par. 


stones 


19. 

20, 

!9- 

19. 

!9. 

20, 

196,  199 

205 ,  206,  216 


190 

207 

ig8 

J97 

196 

210 


testing . Art.  19,  Par.  196,  201;  Art.  20,  Par.  208,  209,  210,  2ti,  212 

weight  of . Art.  19,  Par.  197;  Art.  20,  Par.  215 

Coffer-dams: 

clay  puddle  for .  Art.  30,  Par.  15 

of  earth .  . .  29'  ^ 

excavation  in .  Art.  30,  Par.  12, 

expenses  of,  uncertain .  Art.  30,  Par. 


13. 

16 


masonry  built  in . 

pumping  out . 

piles  driven  inside . 

timber,  ordinary  construction  of.. 
“  single  walls.  “ 

timber,  on  caissons . 

“  “  “  construction  of 

with  inside  cribs . 

when  used . 

Concrete, 

broken  stone  and  gravel  in . 

composition  of . 

crushing  strength  of . 

exact  proportions . 

general  rules  in  making 


30,  ‘ 

■  18 

30,  ‘ 

*  1 1 

30.  ‘ 

■  17 

30.  ‘ 

‘  7.  8 

30,  ' 

‘  9 

49.  ‘ 

‘  3r> : 

3'.  ‘ 

■  20 

30,  ‘ 

‘  13. 1 

30.  ‘ 

‘  19 

2,  ‘ 

‘  24,  : 

2,  ‘ 

‘  22 

3.  * 

‘  38 

2  * 

‘  33 

2,  ‘ 

‘  31 

14 


kind  of  stone  used .  Art.  2,  Par.  27,  29.  30 

mixing .  “  2-  “  23.  26.  27,  28 

proportions  in . Art.  2,  Par.  24,  26,  27,  28,  32;  Art.  49.  Par.  29 

spread  of  base  by .  Art.  3,  Par.  34,  Art.  5  .  Par.  7 


INDEX. 


403 


Concrete: 

under  Washington  Monument .  Art.  2,  Par.  31^ 

usesof .  “  3.  “  34.35 

“  “  under  water . Art.  20,  Par.  217,  218  ;  Art.  47,  Par.  7 

cu.  yds.  per  barrel  of  cement . Art.  2,  Par.  31^,  32 

Cost  of  work: 

brick,  stone,  concrete,  earth . Art.  25,  Par.  267,  270;  Art.  26,  Par.  278 


caissons .  Art.  25,  Par. 


269,  270 
276,  277 

272,  277 
268,  270 
286,  2S7 
284,  285 
282,  283 

273.  274 


quarrying .  .  “  26, 

stone  cutting . . .  “  25, 

trestles .  “  25, 

Ohio  River  Bridge .  27, 

Schuylkill  River  Bridge .  “  27, 

Susquehanna  River  Bridge .  Art.  27,  Par.  281, 

by  contractors .  “  25,  “ 

tables  of  costand  quantities.  Art.  26,  Par.  275,  278;  Art.  27,  Par.  279,  280-88 

remarks  on .  Art.  25,  Par.  266 

Cribs,  ordinary: 

construction  of .  Art.  29,  Par.  3,  4 

sinking,  method  of .  “  29,  “  3,  4,  5 

sunk  on  rock .  “  29,  “  5 

filling  broken  stone  with  grout .  “  47,  “  5 

Cribs  for  deep  foundations: 

combined  with  pneumatic  caisson . Art.  51,  Par.  48,  49,  50,  51,  52 

construction  of .  Art.  47-  Par.  2 

defined . * . ,..  “  47,  “  1 

designs  for .  “  47,  “  4 

“  “  of  timber .  “  47,  “  5 

“  of  iron . Art.  47,  Par.  6;  Art.  49,  Par.  28 

“  “  on  caissons .  Art.  49,  Par.  29 


sinking,  method  of . 

sinking,  difficulties  of . 

when  used . 

examples  of — 

for  Cairo  Bridge . 

“  Havvkesbury  Bridge. 
“  Poughkeepsie  “ 

“  Susquehanna  “ 

“  Schuylkill  “  . 


47, 

47, 

47, 

49, 

47, 

47, 


3 

7 

3 

29 

6 

5 


.  Art.  49,  Par.  30,  31,  32.  33,  34,  36,  37,  38 

. Art.  49,  Par.  30,  36,  37,  38 

Lighthouse,  Diamond  Shoal . . .  Art.  49,  Par.  28 

Remarks .  “  51,  “  53 

Culvert: 

box  and  arch . Art.  18,  Par.  187,  188,  189;  Art.  24,  Par.  251,  252,  253 

waterway  in .  Art.  23,  Par.  245,  246,  247,  248 

pipe... . . .  .  “  23,  “  249,250 


4C4 


INDEX. 


Cutting  and  dressing  stones: 

chisel  draft . . . Art.  6,  Par.  62  to  67 


gauging  stones .  Art. 

method  of .  << 

pitch  line .  << 

requirements .  *< 

shapes,  required . . . 


6,  Par.  67 


Art.  6,  Par.  62, 


64 
66 

65 

62 

63 


23,  24 
24 


tools  used . . 

Cylinders: 

cushing,  piers  with  piles . .  “  g2,  ‘ 

piers,  without  piles . . .  Art.  32 

of  brick  and  concrete.  .Art.  55,  Par.  75;  Art.  57,  Par.  85-87  ;  also  see  Suppl’t 

°f  'ron . Art.  47;  Par.  9;  Art.  49,  Par.  28 

Definition  of  terms .  Art.  28,  Par.  289,  290,  291,  292 

Derricks .  “  28  “  293 

Formulae  : 

for  foundation  projections . . . Art.  56,  Par.  76 


beams  and  stringers.  . 
joints  and  fastenings, 
long  timber  struts.  .  .  . 
driving  piles . 


4C 

40, 

41, 

42, 

43, 

46, 

9- 


70,  71,  72 
60 

68,  73 

9i 

92,  93 
136 

95 


building  on  soft  soils . 

for  wind  pressure . . . . 

retaining-walls . Art.  11,  Par.  109,  no,  1x1 

depth  of  keystones  of  arches .  Art.  14,  Par.  135 

abutments  of  arches .  Art.  24,  Par.  256,-257,  258 

“  “  “  .  “  24,  “  259,  260-265 

Foundation-beds  : 

classification . .  .  Art.  1,  Par.  1  to  19 

conclusions .  “  1,  “  14,  15 

failure  of .  “  1,  “  19 

general  principles  of .  Art.  1,  Par.  1,  2,  3,  4,  5,  6,  7,  8 

pressure  on .  “  1,  “  10,  11,  12,  13 

Tables  of . Art.  1,  Par.  16,  17 

tests  of .  “  1,  “  18 

Foundations,  construction  of : 
by  concrete — see  Concrete. 

“  caissons  or  cribs — see  Caissons  and  Cribs. 

“  timber,  ordinary .  Art.  29,  Par.  2 


“  cribs  “  . 

“  “  “  sinking . 

in  coffer-dams . 

by  “  “  — see  Coffer-dams, 

masonry — see  Masonry  ;  also. . . , 
in  quicksand . 


29, 

29. 

30, 

3, 

55, 


3 

3, 

18 


4,  5 


36,  37 
70-75 


INDEX. 


405 


Foundations,  construction  of: 

for  high  buildings .  Art.  56,  Par.  76,  79;  Art.  57,  Par.  S1-S7 

“  trestles — see  Trestles;  also . .  Art.  41,  Par.  65 

“  freezing  process .  “  54,  “  67,  68,  69 

“  injecting  cement  into  sand . Art.  47,  Par.  5;  Art.  55,  Par.  73 

.  Art.  2,  Par.  20 


definition  of. 


settlement  of . 

“  2 

of  masonry . 

.  "  56, 

“  76 

“  steel  and  concrete . 

“  76 

“  timber  and  concrete.  .  . 

“  76 

remarks . . 

“  81-84 

by  the  Harris  process.  .  .  . 

“  73 

Grout : 

defined . 

.  “  7. 

“  82 

in  gravel  and  sand  under  water 

.  “  47. 

“  5 

“  “  “  “  “ 

.  “  55. 

“  73 

“  masonry.  ...  * . 

.  “  7. 

“  82 

Ice  : 

pressure  and  strength  of.. 

Art.  9,  Par.  94,  95;  “  22, 

“  229-244 

Iron  bolts  : 

strength  of . 

“  60 

Joints  and  fastenings  : 

in  carpentry . 

54.  55.  56 

“  “  . . 

in  masonry . 

principles  of . 

relations  between . 
Lighthouse  : 

iron  crib . 

Lime,  quick . 


Art.  7,  Par.  So 


40, 

40, 

49. 

19. 

20, 


‘  64 
‘  60 

‘  28 
‘  200 
‘  203 


Location  of  piers  : 

base-lines . . . 

steel  tapes  and  base  bars  for 

steel  wire . 

triangulation  for . 

of  bridges . 

Masonry  : 

ashlar . 

backing  or  filling  in . 

a  it  a  tc 

batter  on . 

block-in-course . 

bond  in . 

classification  of . 

dry  stone  . . 


“  53.  “  59 

“  53.  “  59 

“  53.  “  60 


53.  “  59 


Art.  53.  Par. 

61,  62, 

63,  64 

79 

....  “  5, 

“  59 

....  “  7. 

“  79. 

S1-S4 

....  “  7, 

“  96 

-  “  7, 

“  77 

....  “  7. 

“  74, 

73 

....  “  7. 

“  68 

....  “  7, 

“  72 

index: 


406 


Masonry  : 

brick . 

expansion  of .  . 

facing  stones  in . .  .  Art.  5,  Par.  60; 

footing-courses .  Art  3,  Par.  36; 

granite . 

grout  in  . 

header  and  stretcher — see  Definition  of  Terms.  „ . . . . 

inspection  of . 

joints  in . 

laying . 

limestone . 

neat  line  in .  .  .  . . 

rubble,  rough . . . 

“  coursed . 

“  limestone . 

“  sandstone . 

sandstone . 

Masonry  : 

appearance  on  the  face . 


coping . 

piers,  form  of . 

“  stability  of . 

“  pressures  on . 

“  “  “  of  ice  and  drift. 

of  retaining-vvalls . 

starlings  or  cutwaters . 

string-courses  in . 

principles  and  rules  of  construction 


Art.  3, 

Par.  34,  36 

“  5, 

“  59 

“  6, 

“  65,  67 

“ 

“  86 

“  7, 

“  69 

“  7. 

“  82 

“  7, 

“  89 

“  5, 

“  61 

“  7, 

“  80 

“  7, 

“  81,  85,  89 

“  7, 

“  70 

“  7, 

“  84. 

“  7. 

“  73 

“  7. 

“  74,  75,  76 

“  4, 

“  47,  48 

“  4. 

“  41^-46 

“  7, 

“  71 

“  7, 

“  87,  88,  89 

“  8, 

“  90 

“  7, 

“  88 

“  8, 

“  90-93 

“  22, 

“  229 

“  22, 

“  230-244 

“  22, 

“  233-244 

“  10, 

“  103,  104 

“  8, 

“  92,  93 

“  7, 

“  88 

“  18, 

“  190-195 

Mortar  : 

crushing  and  tensile  strength . Art.  19 

cement,  mixed  with  lime . 

“  hardening . 

“  proportions . 

defined  . .  . 

freezing  of . 

lime,  proportions . 

“  hardening... . 

pointing . 

quantity  in  masonry . 

salt  in . 

sand  in . 

yield  per  barrel . 

water  in . 

under  water . 


,  Par.  201;  Art.  20,  Par.  21 1,  212 

.  Art.  20,  Par.  202 

.  “  20,  “  213,  214 

.  “  20,  202,  212 

. .  “  20,  “  202 

.  Art.  20,  Par.  222,  223,  224,  225 


Art 

20, 

Par. 

204 

4  ( 

20, 

213 

4  4 

20, 

4  < 

226 

4  1 

*9, 

(  4 

200 

4  4 

20, 

44 

202 

4  4 

20, 

4  4 

225 

1  4 

21, 

227, 

228 

<  4 

*9> 

200 

4  4 

19* 

200 

<  ( 

20, 

“ 

21-7. 

218 

INDEX. 


407 


Piers : 

all  iron. 


screw-piles . 


construction  of. 


depth  sunk . 

sinking  by  turning. 


cushing  cylinder. 


construction  of . 

stability  of.  . .Art.  22,  Par.  229-244; 


masonry. 


pressures  on., 
stability  of.  . .  . 
dimensions  of. 


timber . 

“  construction  of . 

Piles  : 

alignment  in  driving . . 

cutting  off  below  water  by  divers . 

“  “  “  “  “  machinery. 


Art. 

52, 

Par. 

54-58 

‘  ‘ 

52, 

‘  4 

55 

*  ' 

52, 

4  1 

56,  57 

t  1 

52, 

‘  ‘ 

57 

1  * 

52, 

44 

58 

;  also  see  Supplement 

Art. 

53, 

Par. 

59 

32, 

t  i 

23,  24 

‘ ‘ 

32, 

i  i 

23 

32, 

>  t 

23,  24 

** 

7, 

68-93 

if 

9, 

t  f 

94,  95 

*  ‘ 

9> 

t  i 

95 

<< 

9, 

4  f 

96 

* 4 

27, 

l  4 

279,  288 

1 1 

34, 

4  4 

28 

34, 

4  4 

28,  29 

if 

45, 

4  4 

121,  122 

<  i 

45, 

4  4 

115 

** 

45, 

<  4 

Il6,  117 

“  in  quicksand . 

determining  bearing  power. 


bearing  power  to  be  determined  by  experience  and 

experiment . 

formulae  for  driving . 

“  “  “  latest . . 

discussion  of  formulae . 

holding  piles  in  position . 

frictional  resistance  and  direct  support,  dependent 

on . 

bearing  power,  examples  of . 

“  “  “  “ . Art.  56,  Par.  77;  also  see  Supplement 

frictional  resistance  of . Art.  42,  Par.  84,  85,  86 


Par. 

94,  95,  96,  97 

45,  Par.  108 

42, 

“  89 

42, 

“  84 

43, 

“  93 

42, 

“  83,  84 

42, 

“  9i 

43, 

“  92 

43. 

“  92,93,931 

45, 

“  124 

43, 

“  92,93,931 

42, 

“  84-87 

injury  to,  in  driving .  Art.  42,  Par.  81,  82,  87; 

iron  shoes  for . 

for  coffer-dams . 

inside  of  coffer-dams . 

knowledge  of  strata  important . Art.  42,  Par.  8 

preparing,  for  driving  . Art.  42,  Par.  79 

pointing  piles . .  “  42,  “  79 


“  56,  ‘ 

‘  79 

“  56,  ‘ 

‘  77 

“  42,  ‘ 

*  80 

“  30,  ‘ 

‘  7,  8 

“  30,  ‘ 

‘  17 

1;  also  see  Supplement 


408 


INDEX. 


Piles  : 

purposes  of  driving .  Art  44,  Par.  99,  101-104 

driving  in  rock  beds .  Art.  44,  Par.  105,  106 

sinking  by  water-jet .  .Art.  43,  Par.  98;  also  see  Supplement 

hand .  Art.  43,  Par.  92,  96 

sand . 

setting  piles  in  the  leads . 

for  trestles . 

“  “  description  and  construction . 

three  and  four  pile-bents . 

relative  cost  of  construction . 

temporary  trestles . 

for  wharves,  dikes,  and  jetties . 

under  open  caissons . 

“  timber  piers . . . 

unreliability  of  formulae . 

Pozzuolana . Art.  20,  Par.  219,  220,  221 

Pumps  : 

mud  and  sand . Art.  49»  Par.  29;  Art.  50,  Par.  41 

force  and  centrifugal . 

Quicksand  : 

defined . 

foundations  in .  Art 

against  walls . . 

Quarrying  : 

general  principles . 

blasting  in .  . . . 

drilling  holes  by  hand . 

“  “  machinery .  .... 

explosives . 

economy  in . 

Resistances  : 

frictional . 

“  determination  of . 

on  piles .  Art.  43,  Par.  92,93,934; 

Retaining-walls  : 

as  reservoir  walls  or  dams . 

formulae  for  thickness  of . . 

frictional  stability . 

masonry  in . 

pressure  of  earth  against . 

stability  against  overturning . 

theory  of . 

thickness  of . 

surcharged . 


44, 

100 

45, 

“  123 

44, 

“  104 

44, 

“  104 

44, 

“  104 

44, 

“  104 

45, 

“  107 

44, 

“  102, 103 

31, 

“  21 

34, 

“  28,  29 

42, 

00 

,  Par.  219,  220,  221 

■  50, 

Par.  41 

30, 

“  n 

55. 

“  70,  7i 

Par.  66,  67,  68,  69 

55> 

Par.  70,  72-75 

11, 

“  US 

5, 

53 

5, 

“  54 

5, 

“  55 

5, 

“  56 

5, 

“  57  ' 

5, 

“  59,  60,  61 

56, 

“  77.  73,  79 

56, 

“  78 

56, 

“  79 

11, 

“  ill 

11, 

“  109-m 

10, 

“  107,  108 

10, 

“  103,  104 

10, 

“  97,  9s,  99 

10, 

“  100 

11, 

“  109 

10, 

“  102 

24. 

“  259,  260 

INDEX. 


409 


Retaining-walls  : 

surcharged .  Art. 

to  support  quicksand  or  mud .  “ 

Sand  : 

uses  of .  “ 

proportions  of  .  “ 

size  of  grain .  “ 

qualities  of  good .  “ 

Soundings  and  borings,  method,  purposes,  and  impor¬ 
tance  of .  Art.  33,  Par.  25,  26,  27; 

Stones  : 

for  building,  classified .  Art. 

granite . “ 

limestone .  “ 

sandstone .  “ 

examination  of .  “ 

two  grades  of .  “ 

strength  of .  “ 

absorption  of  water  by .  “ 

expansion  of,  with  heat .  “ 

tests  of  strength .  “ 

Stress  and  strains  : 

in  timber .  “ 

in  iron .  “ 

Swamps  : 

earth  embankments  on .  “ 

crust  of,  not  to  be  broken .  “ 

building  on .  “ 

“  formula  for .  “ 

embankments,  ordinary .  “ 

“  “  cost  of .  “ 

Tables  : 

of  long  spans  and  depth  sunk  of  some  large  bridges.  “ 

of  timber,  strength  of .  “ 

of  stones  and  earth,  strength  of .  “ 

mortar,  strength  of .  “ 

cost  of  work,  and  quantities .  “ 

“  “  quarrying .  “ 

“  “  Susquehanna  River  Bridge .  “ 

“  “  Schuylkill  “  “  .  “ 

“  “  Ohio  River  Bridge .  “ 

stones,  resistance  to  crushing .  “ 

“  “  “  cross-breaking .  Art.  28,  Par 

tensile  strength  of  mortar .  Art. 

adhesive  “  “  “  .  “ 

absorptive  power .  “ 


1 1,  Par.  1 12 


II, 

“  113 

21, 

“  227,  228 

21, 

“  227,  228 

21, 

“  227,  228 

21, 

“  227,  22S 

also  see  Appendi 

•  4, 

Par.  39,  40 

4. 

“  4i 

4. 

“  47,  43 

4, 

“  4^-45 

4, 

“  42,  43,  * 

4. 

“  46 

4. 

“  46 

4, 

“  50 

4, 

“  50 

27. 

“  2S7 

4h 

“  67-74 

40, 

“  60 

46, 

“  125-129 

46, 

“  135 

46, 

“  135 

46, 

“  135 

46, 

“  130 

44. 

“  133 

53, 

“  65 

41. 

“  72 

X, 

“  16,  17 

19, 

“  201 

26, 

275,  27S 

26, 

“  276 

27, 

“  279-2S3 

27, 

“  2S4,  2S5 

27, 

“  286,  287 

23, 

“  294-297 

.  294,  295,  29S,  29 

28, 

Par.  300 

28, 

“  301 

28, 

“  302 

4io 


INDEX. 


Tables  : 

expansion  and  contraction . 

specific  gravity . 

angles  o£  repose . 

bearing  power  of  soils . 

Timber . 

description  of . . 

general  properties . 

life  of . . . 

examination  for  rot . 

preservation  of .  Art.  38,  Par.  43,  44,  45 

durability  of . . 

“  “  constantly  wet .  . 

protecting  joints  in . . . 

strength  of . 


strains  in  . 

table  of  strength . 

Trestles  : 

framed,  of  timber . 

types  and  construction  of. 


Art. 


28,  Par.  303 

28,  ‘ 

‘  304 

28,  ' 

305 

2S,  ‘ 

‘  306 

29,  ‘ 

‘  1 

36,  ‘ 

35-39 

36, 

'*  40 

37.  ' 

4i,  42 

37.  ‘ 

‘  42 

46;  see  also  Appendix 


spans  over  1 5  ft . 

timber  used  in . 

joints  in  . 

braces .  . 

under  trussed  stringers . 

over  “  “  . 

built  beams  for  “  . 

foundation-beds  for . . 

stresses  in  members . 

strength  of  “  . 

determining  dimensions . 

bill  of  materials  for . 

comparative  cost  of  pile  and  framed . 

economical  span  in .  Art 

terms  of  payment  for . 

hints  on  designing . 

Wind,  pressure  of . 


Art.  38, 

Par.  47 

“  38, 

“  48 

“  38. 

“  49 

“  40, 

“  63 

“  41. 

“  72,  73 

“  4>, 

“  67-71 

“  4i. 

“  72 

“  4i. 

“  72 

“  35. 

“  30,31. 

“  35. 

“  30,  31, 

“  39. 

“  50,  51 

“  41. 

“  66 

“  35. 

“  33.  34 

“  35. 

“  34 

“  40, 

“  53-62 

“  40, 

“  58 

“  35. 

“  33 

“  40, 

“  64 

“  40, 

“  64 

“  4i. 

“  65 

“  41, 

“  67 

“  4i. 

“  68,  6g 

“  4i. 

“  68,  69, 

“  41. 

“  74 

45.' 

“  109 

:.  45,  Par.  no,  in.  1 

Art.  45. 

Par.  113 

“  45. 

“  K4 

“  9- 

“  94,  95 

Plate  I. 

Pier  No.  5,  Ohio  River  Bridge,  Point  Pleasant,  W.  Va. 


High  Wate 


Fig.  2. 


-AH.BKiWIE  CC..N.Y. 


SIDE  V  F  E  Wvj 


END  VIEW. 


plan  and  horizontal  sections. 


Plate  II. 


Ends  Large  Stone.  Fig.  i.  Circular  Backing. 


Pointed-end  Concrete  Backing. 


Fig.  2. 


Triangular-end  Rubble  Backing. 


Plate  III. 

Coffer-dam  with  Inner  Ofen  Caisson  or  Crib,  Ohio  River  Bridge,  Foint  Pleasant,  W.  Va. 


Fig.  i. 


•  Fig.  2. 


Fig.  4. 

E W  OF  IN.NER  CAISSON 


Plate  IV. 


Plate  V. 


Plate  Va. 

Derrick  on  Top  of  Pier  and  raised  with 


Scale  vl,to 


Fia  a. 


Plate  Vi. 


ORDINARY  FRAMED  TRESTLE 
Fig.  4.  « 

r-iA-V. 4 


PILE  TRESTLES. 


Fig.  6. 


0= *  12'  6  'Mil. .  Q  AM. CK. NOTE  C0\N^Jfi 


GENERAL  PLAN  OF  DECK. 


Plate  VII. 

Sips  Elevations. 


Plate  VIII, 


Details  of  Joints,  Built  Beams,  etc. 


Plate  IX. 

Four-story  Trestle-eent,  Shenandoah  Valley  Railway.  W.  W.  Coe,  Chief  Engineer. 


Pi. ate  X. 

Caisson  for  Diamond  Shoals  Lighthouse.  Designed  by  Anderson  and  Bark. 

Fig.  i. 


Fig.  i. — Vertical  Section. 
Fig.  2. — Horizontal  Sections. 


Plate  XI. 

Combined  Crib  and  Pneumatic  Caisson. 


Fig.  i.— Vertical  Longitudinal  Section. 

Fig.  2.  Horizontal*  Sections  at  Several  Points. 


Plate  XII. 

Combined  Crib  and  Pneumatic  Caisson, 


Tig.  4. 


Fig.  3. — Part  Vertical  Section,  Piles  Driven  in  Interior. 

Fig.  4. — Part  Vertical  Section,  Sunk  below  Pneumatic  Limit. 


Plate  XIII. 


Pneumatic  Caisson,  Crib  and  Coffer-dam,  Susquehanna  River  Bridge,  B.  &  O.  Ry. 


Fig.  2. 
end  view 
,  A. 

»*’?  .  31.  1" 

p  6  


Fig.  t. 

SIDE  VIEW 

B. 

77'  715" 


Total  Timber  in  Caisson  316,689,50  Ft.B.M. ' 
v  j>  >}  Crib  „s  3-43,993,14  « 

[v  'y  Coffer-Dam.  1 08,51  V, 61 

i  569,200,25  ” 


Displacement  sinking  Caisson  156,183,07  cub.ft. 

'-it..  '  excavation  below_cuttiDg  edge  3,405,24  » 


Fig.  3. 

PLANS 

c. 

PLAN  OF  CRIB 


i  j 

!  1 

!  1 

jl 

„  i 

!l 

j! 

IT  1 

1  1 

- J.L - 

_ 11m 

j  l 

.  _  _ 

- IT — 

ir  1 1 1 

-12-X-12-J - 

!  0 

|p 

!! 

1  |  . 

|  | 

!  | 
ii 

A 

A 

A 

A 

IV 

V 

PLAN  OF  COFFER-DAM  AND  SYSTEM  OF.  BRACING- 

Fig,  4. 


Plate  XIV, 


ELEVAT10.M  OF  OCTAGONAL  CAISSON 

Scale 2  feet. 


^Timber- in  Caisson 
211,289  Ft.B.M, 


Fig.  i.  Part  Elevation,  Part  Longitudinal  Section,  showing  Interior  Details,  Shafts,  Pipes,  in  Position. 


Plate  XV, 

jOetails  of  Construction,  Octagonal  Caisson. 


FLAM  SHOWING 
ECTTCM  CHORD  &.  BRACES 


quantities:, 

Square  Timber  211,845.35  Ft.  B.  M.  ,  Concrete  107.23  Cub.„Yds. 
Plank 


f  20,462.73  ”  ’’  ’’  Excavation  116,084.96  Cub.  Feet} 

Total  Iron  58,61S  lbs. 


DIAGRAM  SHOWING  DISTRIBUTION  OF  SCREW  BOLTS 
AND  DRIFT  BOLTS  IN  VERTICALS  OF  OCTAGONAL  AND 
, _  RECTANGULAR  CAISSONS. 

2  Screw  Bolts  41  ins.  3  Screw  .Bolts  41  ins. 


Scale  of  Feet 


Plate  XV  i. 

Square  Caisson  for  Masonry  Abutment. 


Fig.  3* 


12X32 


L A 

\H  !  1 

12x  12" 

A-L 

|  | 

1 

Long 

tudinal 

12xi2  "Si 

H  7/ 

px)2 

12X12"  js 

- - . — 1 

*==r- 

~~ 

SECTION 
6HOWING  ARRANGEMENT  A 
OF  BOLTS 
Scale  %—b 


12  x  12 

TTTTTT 

:  11  1  1  1  1  1  M  11  $[2;  1 . 1  11  11  11  11  1  n  1  1  1  1 1 

12  X  12' 

1  II  II 

1  1  1  1  1  1  11  i  111  wmi  11:1111111111111 

1  II 

TITfTM  mTT  1  II  1  1  II  1 

2 

12  x 12  ;  a 

T^rr 

1  1  II  I  II  1  1  T 1  1  W  Cl  |  |  |  ;  ||  ||  |  |  |  |  |  |  |  | 

12  x  12' 

1 LL 

1  1  1  1  1  1  1  i^r  1  1  1  1  1  1  m  i  11' 

W  \  :  12x12'  /  r  r- 

Fig.  i.— Cross-section. 


Fig.  2. — Longitudinal  Section* 


Plate  XVII. 


Horizontal  Sections  Octagonal  Caisson  for  Pivot  Pier,  Schuykill  River  Bridge, 
B.  &  O.  Rv.  Showing  Details  of  Construction. 


Fig.  i. 


Open  TVatfs lillecl  with  Concrete. 


Plate  XVUI. 

General  Designs  Pneumatic  Caissons. 

Figs,  i,  2,  3  and  4  used  in  Foundations  Bismarck  and  Cairo  Bridges.  Open  Crib-work  built  to  any  desired  Heigri. 

Fig.  2. 


LONGITUDINAL  section,  showing  walls  with  solid  timber,  open  crib  roof 

Fig.  5. 


FIGS.  1,  2,  3,  i  &  5  show  general 
design  of  Caissons  with 
shaped  walls  and  cutting  edge, 
with  open  crib  above;  air’ locks 
flea”  bottom  often  used. 
_Designed  by  G.  S.  MORRISON, 
T  Chief  Engineer. 


Fig.  4. 


TOP  VIEW  &  HORIZONTAL  SECTION. 


Fig.  6. 

CROSS  SECTION 
PNEUMATIC  CAISSON 
PLATTSMOUTrt'S  BRIDGE 


Plate  XIX. 


Design  of  Pneumatic  Caisson  and  Crie  with  Pointed  End;  also,  Lower  Part  of  Masonry  Pier  with  Cutwater 


Plate  XX, 


Air-lock,  Shaft,  Pipe  ,  and  Detaii.s. 


Fig.  6. 

Supply-shaft. 


Plate  XXI. 


Screw-file  Pier,  Mobile  River,  L.  &  N.  Ry. 


Plate  XX3I. 


Daily  Progress  Sinking  Octagonal  Caisson, 


